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LM2731
SNVS217G –MAY 2004–REVISED SEPTEMBER 2015
LM2731 0.6/1.6-MHz Boost Converters With 22-V Internal FET Switch in SOT-23
1 Features 3 Description
The LM2731 switching regulators are current-mode
1• 22-V DMOS FET Switch boost converters operating at fixed frequencies of 1.6
• 1.6-MHz (X Option), 0.6-MHz (Y Option) Switching MHz (X option) and 600 kHz (Y option).
Frequency The use of SOT-23 package, made possible by the
• Low RDS(ON) DMOS FET minimal power loss of the internal 1.8-A switch, and
• Switch Current Up to 1.8 A use of small inductors and capacitors result in the
• Wide Input Voltage Range (2.7 V to 14 V) highest power density of the industry. The 22-V
internal switch makes these solutions perfect for
• Low Shutdown Current (<1 µA) boosting to voltages up to 20 V.
• 5-Lead SOT-23 Package These parts have a logic-level shutdown pin that can
• Uses Tiny Capacitors and Inductors reduce quiescent current and extend battery life.
• Cycle-by-Cycle Current Limiting Protection is provided through cycle-by-cycle current
• Internally Compensated limiting and thermal shutdown. Internal compensation
simplifies design and reduces component count.
2 Applications Device Information(1)
• White LED Current Sources PART NUMBER PACKAGE BODY SIZE (NOM)
• PDAs and Palm-Top Computers LM2731 SOT-23 (5) 1.60 mm × 2.90 mm
• Digital Cameras (1) For all available packages, see the orderable addendum at
• Portable Phones and Games the end of the data sheet.
• Local Boost Regulators
Block Diagram
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
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Table of Contents
1 Features.................................................................. 18 Application and Implementation ........................ 13
8.1 Application Information............................................ 13
2 Applications ........................................................... 18.2 Typical Application.................................................. 13
3 Description............................................................. 18.3 System Examples ................................................... 18
4 Revision History..................................................... 29 Power Supply Recommendations...................... 20
5 Pin Configuration and Functions......................... 310 Layout................................................................... 20
6 Specifications......................................................... 310.1 Layout Guidelines ................................................. 20
6.1 Absolute Maximum Ratings ...................................... 310.2 Layout Example .................................................... 20
6.2 ESD Ratings.............................................................. 310.3 Thermal Considerations........................................ 21
6.3 Recommended Operating Conditions....................... 411 Device and Documentation Support................. 22
6.4 Thermal Information.................................................. 411.1 Device Support...................................................... 22
6.5 Electrical Characteristics........................................... 511.2 Community Resources.......................................... 22
6.6 Typical Characteristics.............................................. 711.3 Trademarks........................................................... 22
7 Detailed Description............................................ 11 11.4 Electrostatic Discharge Caution............................ 22
7.1 Overview................................................................. 11 11.5 Glossary................................................................ 22
7.2 Functional Block Diagram....................................... 11 12 Mechanical, Packaging, and Orderable
7.3 Feature Description................................................. 11 Information........................................................... 22
7.4 Device Functional Modes........................................ 12
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision F (November 2012) to Revision G Page
• Added ESD Ratings table, Feature Description section, Device Functional Modes,Application and Implementation
section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and
Mechanical, Packaging, and Orderable Information section. ................................................................................................ 1
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J A A
J A
T (MAX) T 125 T
P (MAX) 265
-
--
= =
q
3
FB
2
GND
1
SW
45
SHDN VIN
formula: . If power dissipation exceeds the maximum specified above, the internal thermal
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SNVS217G –MAY 2004–REVISED SEPTEMBER 2015
5 Pin Configuration and Functions
DBV Package
5-Pin SOT-23
Top View
Pin Functions
PIN I/O DESCRIPTION
NAME NO.
FB 3 I Feedback point that connects to external resistive divider.
GND 2 PWR Analog and power ground
SHDN 4 I Shutdown control input. Connect to VIN if the feature is not used.
SW 1 O Drain of the internal FET switch
VIN 5 PWR Analog and power input
6 Specifications
6.1 Absolute Maximum Ratings(1)
MIN MAX UNIT
Operating Junction Temperature –40 125 °C
Lead Temperature (Soldering, 5 sec.) 300 °C
Power Dissipation(2) Internally Limited
FB Pin Voltage –0.4 6 V
SW Pin Voltage –0.4 22 V
Input Supply Voltage –0.4 14.5 V
SHDN Pin Voltage –0.4 VIN + 0.3 V
Storage Temperature, Tstg –65 150 °C
(1) Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
(2) The maximum power dissipation which can be safely dissipated for any application is a function of the maximum junction temperature,
TJ(MAX) = 125°C, the junction-to-ambient thermal resistance for the SOT-23 package, RθJA = 265°C/W, and the ambient temperature,
TA. The maximum allowable power dissipation at any ambient temperature for designs using this device can be calculated using the
protection circuitry will protect the device by reducing the output voltage as required to maintain a safe junction temperature.
6.2 ESD Ratings VALUE UNIT
V(ESD) Electrostatic discharge Human body model (HBM), per ANSI/ESDA/JEDEC JS-001(1)(2) ±2000 V
(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
(2) The human body model is a 100-pF capacitor discharged through a 1.5-kΩresistor into each pin.
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6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted) MIN NOM MAX UNIT
VIN Input Supply Voltage 2.7 14 V
Vsw SW Pin Voltage 3 20 V
Vshdn Shutdown Supply Voltage(1) 0 VIN V
TJJunction Temperature Range –40 125 ºC
(1) This pin should not be allowed to float or be greater than VIN + 0.3 V.
6.4 Thermal Information LM2731
THERMAL METRIC(1) DBV (SOT-23) UNIT
5 PINS
RθJA Junction-to-ambient thermal resistance 209.9 °C/W
RθJC(top) Junction-to-case (top) thermal resistance 122 °C/W
RθJB Junction-to-board thermal resistance 38.4 °C/W
ψJT Junction-to-top characterization parameter 12.8 °C/W
ψJB Junction-to-board characterization parameter 37.5 °C/W
RθJC(bot) Junction-to-case (bottom) thermal resistance N/A °C/W
(1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
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6.5 Electrical Characteristics
Limits are for TJ= 25°C. Unless otherwise specified: VIN = 5 V, VSHDN = 5 V, IL= 0 A.
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
VIN Input Voltage −40°C ≤TJ≤125°C 2.7 14 V
VIN = 2.7 V 7
−40°C ≤TJ≤5.4
125°C
RL= 43 ΩVIN = 3.3 V 10
X Option(3)
−40°C ≤TJ≤8
125°C
VIN = 5 V 16
VIN = 2.7 V 7.5
−40°C ≤TJ≤6
125°C
RL= 43 ΩVIN = 3.3 V 11
Y Option(3)
−40°C ≤TJ≤8.75
125°C
VIN = 5 V 15
VOUT (MIN) Minimum Output Voltage Under Load V
VIN = 2.7 V 5
−40°C ≤TJ≤3.75
125°C
RL= 15 ΩVIN = 3.3 V 6.5
X Option(3)
−40°C ≤TJ≤5
125°C
VIN = 5 V 10
VIN = 2.7 V 5
−40°C ≤TJ≤4
125°C
RL= 15 ΩVIN = 3.3 V 7
Y Option(3)
−40°C ≤TJ≤5.5
125°C
VIN = 5 V 10
TJ= 25°C 1.8 2
ISW Switch Current Limit See(4) A
−40°C ≤TJ≤1.4
125°C
TJ= 25°C 260 400
ISW = 100 mA −40°C ≤TJ≤
Vin = 5 V 500
125°C
RDS(ON) Switch ON-Resistance mΩ
TJ= 25°C 300 450
ISW = 100 mA −40°C ≤TJ≤
Vin = 3.3 V 550
125°C
−40°C ≤TJ≤
Device ON 1.5
125°C
SHDNTH Shutdown Threshold V
−40°C ≤TJ≤
Device OFF 0.5
125°C
VSHDN = 0 0
TJ= 25°C 0
ISHDN Shutdown Pin Bias Current µA
VSHDN = 5 V −40°C ≤TJ≤2
125°C
TJ= 25°C 1.230
VFB Feedback Pin Reference Voltage VIN = 3 V V
−40°C ≤TJ≤1.205 1.255
125°C
TJ= 25°C 60
IFB Feedback Pin Bias Current VFB = 1.23 V nA
−40°C ≤TJ≤500
125°C
(1) Limits are ensured by testing, statistical correlation, or design.
(2) Typical values are derived from the mean value of a large quantity of samples tested during characterization and represent the most
likely expected value of the parameter at room temperature.
(3) L = 10 µH, COUT = 4.7 µF, duty cycle = maximum
(4) Switch current limit is dependent on duty cycle (see Typical Characteristics).
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Electrical Characteristics (continued)
Limits are for TJ= 25°C. Unless otherwise specified: VIN = 5 V, VSHDN = 5 V, IL= 0 A.
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
TJ= 25°C 2
VSHDN = 5 V, Switching −40°C ≤TJ≤
"X" 3
125°C mA
TJ= 25°C 1
VSHDN = 5 V, Switching −40°C ≤TJ≤
"Y"
IQQuiescent Current 2
125°C
TJ= 25°C 400
VSHDN = 5 V, Not −40°C ≤TJ≤
Switching 500 µA
125°C
VSHDN = 0 0.024 1
ΔVFB/ΔVIN FB Voltage Line Regulation 2.7 V ≤VIN ≤14 V 0.02 %/V
TJ= 25°C 1.6
“X” Option −40°C ≤TJ≤1 1.85
125°C
FSW Switching Frequency(5) MHz
TJ= 25°C 0.6
“Y” Option −40°C ≤TJ≤0.4 0.8
125°C
TJ= 25°C 86%
“X” Option −40°C ≤TJ≤78%
125°C
DMAX Maximum Duty Cycle(5) TJ= 25°C 93%
“Y” Option −40°C ≤TJ≤88%
125°C
ILSwitch Leakage Not Switching VSW = 5 V 1 µA
(5) Specified limits are the same for Vin = 3.3 V input.
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TEMPERATURE (oC)
OSCILLATOR FREQUENCY (MHz)
0.48
0.5
0.52
0.54
0.56
0.58
0.6
VIN = 5V
VIN = 3.3V
-50 -25 0 25 50 75 100 125 150
95.9
96
96.1
96.2
96.3
96.4
96.5
96.6
96.7
96.8
MAX DUTY CYCLE (%)
TEMPERATURE (oC)
VIN = 5V
VIN = 3.3V
-50 -25 025 50 75 100 125 150
TEMPERATURE (oC)
OSCILLATOR FREQUENCY (MHz)
0.48
0.5
0.52
0.54
0.56
0.58
0.6
VIN = 5V
VIN = 3.3V
-50 -25 0 25 50 75 100 125 150
-50 -25 0 25 50 75 100 125 150
TEMPERATURE (oC)
1.8
1.85
1.9
1.95
2
2.05
2.1
2.15
2.2
IQ VIN ACTIVE (mA)
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
TEMPERATURE (oC)
IQ VIN ACTIVE (mA)
-50 -25 025 50 75 100 125 150
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6.6 Typical Characteristics
Unless otherwise specified: VIN = 5 V, SHDN pin tied to VIN.
Figure 2. IqVIN (Active) vs Temperature - Y Option
Figure 1. IqVIN (Active) vs Temperature - X Option
Figure 4. Oscillator Frequency vs Temperature - Y Option
Figure 3. Oscillator Frequency vs Temperature - X Option
Figure 6. Maximum Duty Cycle vs Temperature - Y Option
Figure 5. Maximum Duty Cycle vs Temperature - X Option
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2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5
VIN (V)
0
50
100
150
200
250
300
350
RDS_ON (m:)
CURRENT LIMIT (A)
2
2.1
2.2
2.3
2.4
2.5
2.6
-40 -25 025 50 75 100 125
TEMPERATURE (oC)
FEEDBACK VOLTAGE (V)
1.222
1.223
1.224
1.225
1.226
1.227
1.228
1.229
1.23
1.231
-40 -25 025 50 75 100 125
TEMPERATURE (oC)
RDS(ON) (:)
-40 -25 025 50 75 100 125
TEMPERATURE (oC)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Vin = 5V
Vin = 3.3V
IQ VIN (IDLE) (PA)
TEMPERATURE (oC)
340
345
350
355
360
365
370
375
380
-50 -25 025 50 75 100 125 150
TEMPERATURE (oC)
FEEDBACK BIAS CURRENT (PA)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
-50 -25 025 50 75 100 125 150
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Typical Characteristics (continued)
Unless otherwise specified: VIN = 5 V, SHDN pin tied to VIN.
Figure 8. Feedback Bias Current vs Temperature
Figure 7. IqVIN (Idle) vs Temperature
Figure 9. Feedback Voltage vs Temperature Figure 10. RDS(ON) vs Temperature
Figure 11. Current Limit vs Temperature Figure 12. RDS(ON) vs VIN
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350
050 100 150 200 250 300
LOAD (mA)
EFFICIENCY (%)
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
0
10
20
30
40
50
60
70
80
90
100
350
050 100 150 200 250 300
LOAD (mA)
400
600
0100 200 300 400 500
LOAD (mA)
EFFICIENCY (%)
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50
0
10
20
30
40
50
60
70
80
EFFICIENCY (%)
LOAD (mA)
1400
0200 400 600 800 1000 1200
LOAD (mA)
EFFICIENCY (%)
0
10
20
30
40
50
60
70
80
90
100
050 100 150 200 250 300
LOAD (mA)
EFFICIENCY (%)
0
10
20
30
40
50
60
70
80
90
100
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Typical Characteristics (continued)
Unless otherwise specified: VIN = 5 V, SHDN pin tied to VIN.
VIN = 2.7 V VOUT = 5 V VIN = 4.2 V VOUT = 5 V
Figure 13. Efficiency vs Load Current - X Option Figure 14. Efficiency vs Load Current - X Option
VIN = 2.7 V VOUT = 12 V VIN = 5 V VOUT = 12 V
Figure 15. Efficiency vs Load Current - X Option Figure 16. Efficiency vs Load Current - X Option
VIN = 2.7 V VOUT = 5 V
VIN = 5 V VOUT = 18 V
Figure 18. Efficiency vs Load Current - Y Option
Figure 17. Efficiency vs Load Current - X Option
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0100 200 300 400 500 600
LOAD (mA)
EFFICIENCY (%)
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
050 100 150 200 250
LOAD (mA)
0
10
20
30
40
50
60
70
80
90
100
020 40 60 80
LOAD (mA)
EFFICIENCY (%)
0
10
20
30
40
50
60
70
80
90
100
1400
0200 400 600 800 1000 1200
LOAD (mA)
EFFICIENCY (%)
0
10
20
30
40
50
60
70
80
90
100
1400
0200 400 600 800 1000 1200
LOAD (mA)
EFFICIENCY (%)
0
10
20
30
40
50
60
70
80
90
100
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Typical Characteristics (continued)
Unless otherwise specified: VIN = 5 V, SHDN pin tied to VIN.
VIN = 4.2 V VOUT = 5 V
VIN = 3.3 V VOUT = 5 V
Figure 19. Efficiency vs Load Current - Y Option Figure 20. Efficiency vs Load Current - Y Option
VIN = 2.7 V VOUT = 12 V VIN = 3.3 V VOUT = 12 V
Figure 21. Efficiency vs Load Current - Y Option Figure 22. Efficiency vs Load Current - Y Option
VIN = 5 V VOUT = 12 V
Figure 23. Efficiency vs Load Current - Y Option
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7 Detailed Description
7.1 Overview
The LM2731 device is a switching converter IC that operates at a fixed frequency (0.6 or 1.6 MHz) for fast
transient response over a wide input voltage range and incorporates pulse-by-pulse current limiting protection.
Because this is current mode control, a 33-mΩsense resistor in series with the switch FET is used to provide a
voltage (which is proportional to the FET current) to both the input of the pulse width modulation (PWM)
comparator and the current limit amplifier.
7.1.1 Theory of Operation
At the beginning of each cycle, the S-R latch turns on the FET. As the current through the FET increases, a
voltage (proportional to this current) is summed with the ramp coming from the ramp generator and then fed into
the input of the PWM comparator. When this voltage exceeds the voltage on the other input (coming from the
Gm amplifier), the latch resets and turns the FET off. Because the signal coming from the Gm amplifier is derived
from the feedback (which samples the voltage at the output), the action of the PWM comparator constantly sets
the correct peak current through the FET to keep the output voltage in regulation.
Q1 and Q2 along with R3 - R6 form a bandgap voltage reference used by the IC to hold the output in regulation.
The currents flowing through Q1 and Q2 will be equal, and the feedback loop will adjust the regulated output to
maintain this. Because of this, the regulated output is always maintained at a voltage level equal to the voltage at
the FB node "multiplied up" by the ratio of the output resistive-divider.
The current limit comparator feeds directly into the flip-flop that drives the switch FET. If the FET current reaches
the limit threshold, the FET is turned off and the cycle terminated until the next clock pulse. The current limit
input terminates the pulse regardless of the status of the output of the PWM comparator.
7.2 Functional Block Diagram
7.3 Feature Description
The LM2731 is a fixed-frequency boost regulator IC that delivers a minimum 1.8-A peak switch current.
The device provides cycle-by-cycle current limit protection as well as thermal shutdown protection. The device
can also be controlled through the shutdown pin.
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7.4 Device Functional Modes
7.4.1 Shutdown Pin Operation
The device is turned off by pulling the shutdown pin low. If this function is not going to be used, the pin should be
tied directly to VIN. If the SHDN function will be needed, a pullup resistor must be used to VIN (approximately 50
kΩto 100 kΩrecommended). The SHDN pin must not be left unterminated.
7.4.2 Thermal Shutdown
Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature
exceeds 160°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature
drops to approximately 150°C.
7.4.3 Current Limit
The LM2731 uses cycle-by-cycle current limiting to protect the internal NMOS switch. It is important to note that
this current limit will not protect the output from excessive current during an output short-circuit. The input supply
is connected to the output by the series connection of an inductor and a diode. If a short circuit is placed on the
output, excessive current can damage both the inductor and diode.
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EFFICIENCY (%)
LOAD CURRENT (mA)
0100 200 300 400
70
80
90
100
500
5 - 12V Boost
^y_sŒ•]}v
LM2731 ³;´
SW
FB
GND
VIN
SHDN
U1
R3
51K
SHDN
GND
5 VIN
C1
2.2PFR2
13.3K
CF
220pF
D1
MBR0520
R1/117K
L1/10PH
C2
4.7PF
12V
OUT
500mA
(TYP)
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8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
The device will operate with input voltage range from 2.7 V to 14 V and provide a regulated output voltage. This
device is optimized for high-efficiency operation with minimum number of external components. For component
selection, see Detailed Design Procedure.
8.2 Typical Application
Figure 24. Application Schematic Figure 25. Efficiency vs Load Current
8.2.1 Design Requirements
The device must be able to operate at any voltage within the recommended operating range. The load current
must be defined in order to properly size the inductor, input, and output capacitors. The inductor must be able to
handle full expected load current as well as the peak current generated during load transients and start-up.
Inrush current at start-up will depend on the output capacitor selection. More details are provided in Detailed
Design Procedure.
The device has a shutdown pin which is used to disable the device. This pin is active-LOW and care must be
taken that the voltage on this pin does not exceed VIN + 0.3 V. This pin must also not be left floating.
8.2.2 Detailed Design Procedure
8.2.2.1 Selecting the External Capacitors
The best capacitors for use with the LM2731 are multi-layer ceramic capacitors. These capacitors have the
lowest ESR (equivalent series resistance) and highest resonance frequency which makes them optimum for use
with high-frequency switching converters.
When selecting a ceramic capacitor, only X5R and X7R dielectric types should be used. Other types such as
Z5U and Y5F have such severe loss of capacitance due to effects of temperature variation and applied voltage,
they may provide as little as 20% of rated capacitance in many typical applications. Always consult capacitor
manufacturer’s data curves before selecting a capacitor. High-quality ceramic capacitors can be obtained from
Taiyo-Yuden, AVX, and Murata.
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8.2.2.2 Selecting the Output Capacitor
A single ceramic capacitor of value 4.7 µF to 10 µF will provide sufficient output capacitance for most
applications. If larger amounts of capacitance are desired for improved line support and transient response,
tantalum capacitors can be used. Aluminum electrolytics with ultra low ESR such as Sanyo Oscon can be used,
but are usually prohibitively expensive. Typical AI electrolytic capacitors are not suitable for switching frequencies
above 500 kHz due to significant ringing and temperature rise due to self-heating from ripple current. An output
capacitor with excessive ESR can also reduce phase margin and cause instability.
In general, if electrolytics are used, TI recommends that they be paralleled with ceramic capacitors to reduce
ringing, switching losses, and output voltage ripple.
8.2.2.3 Selecting the Input Capacitor
An input capacitor is required to serve as an energy reservoir for the current which must flow into the coil each
time the switch turns ON. This capacitor must have extremely low ESR, so ceramic is the best choice. TI
recommends a nominal value of 2.2 µF, but larger values can be used. Since this capacitor reduces the amount
of voltage ripple seen at the input pin, it also reduces the amount of EMI passed back along that line to other
circuitry.
8.2.2.4 Feedforward Compensation
Although internally compensated, the feedforward capacitor Cf is required for stability (see Figure 26). Adding
this capacitor puts a zero in the loop response of the converter. The recommended frequency for the zero fz
should be approximately 6 kHz. Cf can be calculated using the formula:
Cf = 1 / (2 × πX R1 × fz) (1)
8.2.2.5 Selecting Diodes
The external diode used in the typical application should be a Schottky diode. TI recommends a 20-V diode such
as the MBR0520.
The MBR05XX series of diodes are designed to handle a maximum average current of 0.5 A. For applications
exceeding 0.5-A average but less than 1 A, a Microsemi UPS5817 can be used.
8.2.2.6 Setting the Output Voltage
The output voltage is set using the external resistors R1 and R2 (see Figure 26). A minimum value of 13.3 kΩis
recommended for R2 to establish a divider current of approximately 92 µA. R1 is calculated using the formula:
R1 = R2 × (VOUT/1.23 −1) (2)
8.2.2.7 Switching Frequency
The LM2731 is provided with two switching frequencies: the “X” version is typically 1.6 MHz, while the “Y” version
is typically 600 kHz. The best frequency for a specific application must be determined based on the trade-offs
involved:
Higher switching frequency means the inductors and capacitors can be made smaller and cheaper for a given
output voltage and current. The down side is that efficiency is slightly lower because the fixed switching losses
occur more frequently and become a larger percentage of total power loss. EMI is typically worse at higher
switching frequencies because more EMI energy will be seen in the higher frequency spectrum where most
circuits are more sensitive to such interference.
Figure 26. Basic Application Circuit
14 Submit Documentation Feedback Copyright © 2004–2015, Texas Instruments Incorporated
Product Folder Links: LM2731
LOAD
I
1 DC-
0.176A
0.390 µs 0.235 µs
0
Duty Cycle =
VOUT + VDIODE - VIN
VOUT + VDIODE - VSW
LM2731
www.ti.com
SNVS217G –MAY 2004–REVISED SEPTEMBER 2015
8.2.2.8 Duty Cycle
The maximum duty cycle of the switching regulator determines the maximum boost ratio of output-to-input
voltage that the converter can attain in continuous mode of operation. The duty cycle for a given boost
application is defined as:
(3)
This applies for continuous mode operation.
8.2.2.9 Inductance Value
The first question that is usually asked is: “How small can I make the inductor?” (because they are the largest
sized component and usually the most costly). The answer is not simple and involves trade-offs in performance.
Larger inductors mean less inductor ripple current, which typically means less output voltage ripple (for a given
size of output capacitor). Larger inductors also mean more load power can be delivered because the energy
stored during each switching cycle is:
E = L/2 × (lp)2(4)
Where “lp” is the peak inductor current. An important point to observe is that the LM2731 will limit its switch
current based on peak current. This means that since lp(max) is fixed, increasing L will increase the maximum
amount of power available to the load. Conversely, using too little inductance may limit the amount of load
current which can be drawn from the output.
Best performance is usually obtained when the converter is operated in “continuous” mode at the load current
range of interest, typically giving better load regulation and less output ripple. Continuous operation is defined as
not allowing the inductor current to drop to zero during the cycle. All boost converters shift over to discontinuous
operation as the output load is reduced far enough, but a larger inductor stays “continuous” over a wider load
current range.
To better understand these trade-offs, a typical application circuit (5-V to 12-V boost with a 10-µH inductor) will
be analyzed. We will assume:
VIN = 5 V, VOUT = 12 V, VDIODE = 0.5 V, VSW = 0.5 V (5)
Because the frequency is 1.6 MHz (nominal), the period is approximately 0.625 µs. The duty cycle will be 62.5%,
which means the ON-time of the switch is 0.390 µs. When the switch is ON, the voltage across the inductor is
approximately 4.5 V.
Using the equation:
V = L (di/dt) (6)
The di/dt rate of the inductor can then be calculated, which is found to be 0.45 A/µs during the ON time. Using
these facts, what the inductor current will look like during operation can be shown:
Figure 27. 10 µH Inductor Current, 5 V–12 V Boost (LM2731X)
During the 0.390-µs ON-time, the inductor current ramps up 0.176 A and ramps down an equal amount during
the OFF-time. This is defined as the inductor “ripple current”. If the load current drops to about 33 mA, the
inductor current will begin touching the zero axis which means it will be in discontinuous mode. A similar analysis
can be performed on any boost converter, to make sure the ripple current is reasonable and continuous
operation will be maintained at the typical load current values.
Copyright © 2004–2015, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LM2731
IN SW
LOAD SW
DC(V V )
I (max) (1 DC) I (max) 2fL
-
æ ö
= - ´ -
ç ÷
è ø
20 30 40 50 60 70 80 90 100
DUTY CYCLE (%) = [1 - EFF*(VIN / VOUT)]
0
500
1000
1500
2000
2500
3000
SW CURRENT LIMIT (mA)
VIN = 5V
VIN = 3.3V
VIN = 2.7V
VIN = 3V
20 30 40 50 60 70 80 90 100
DUTY CYCLE (%) = [1 - EFF*(VIN / VOUT)]
0
500
1000
1500
2000
2500
3000
SW CURRENT LIMIT (mA)
VIN = 5V
VIN = 3.3V
VIN = 2.7V
VIN = 3V
LM2731
SNVS217G –MAY 2004–REVISED SEPTEMBER 2015
www.ti.com
8.2.2.10 Maximum Switch Current
The maximum FET switch current available before the current limiter cuts in is dependent on duty cycle of the
application. This is illustrated in the graphs below which show typical values of switch current for both the "X" and
"Y" versions as a function of effective (actual) duty cycle:
Figure 28. Switch Current Limit vs Duty Cycle - X Option Figure 29. Switch Current Limit vs Duty Cycle - Y Option
8.2.2.11 Calculating Load Current
As shown in the figure which depicts inductor current, the load current is related to the average inductor current
by the relation:
ILOAD = IIND(AVG) × (1 - DC) (7)
Where "DC" is the duty cycle of the application. The switch current can be found by:
ISW = IIND(AVG) + ½ (IRIPPLE) (8)
Inductor ripple current is dependent on inductance, duty cycle, input voltage and frequency:
IRIPPLE = DC × (VIN-VSW) / (f × L) (9)
Combining all terms, an expression can be developed which allows the maximum available load current to be
calculated:
(10)
The equation shown to calculate maximum load current takes into account the losses in the inductor or turn-OFF
switching losses of the FET and diode. For actual load current in typical applications, we took bench data for
various input and output voltages for both the "X" and "Y" versions of the LM2731 and displayed the maximum
load current available for a typical device in graph form:
16 Submit Documentation Feedback Copyright © 2004–2015, Texas Instruments Incorporated
Product Folder Links: LM2731
2 3 4 5 6 7 8
VIN (V)
0
200
400
600
800
1000
1200
MAX LOAD CURRENT (mA)
VOUT = 5V
VOUT = 8V
VOUT = 10V
VOUT = 12V
VIN (V)
MAX LOAD CURRENT (mA)
0
200
400
600
800
1000
1200
2 3 4 5 6 7 8 9 10 11
VOUT = 5V
VOUT = 8V
VOUT = 10V
VOUT = 12V
VOUT = 18V
LM2731
www.ti.com
SNVS217G –MAY 2004–REVISED SEPTEMBER 2015
Figure 31. Maximum Load Current (Typical) vs VIN - Y
Figure 30. Maximum Load Current (Typical) vs VIN - X Option
Option
8.2.2.12 Design Parameters VSW and ISW
The value of the FET "ON" voltage (referred to as VSW in the equations) is dependent on load current. A good
approximation can be obtained by multiplying the "ON-Resistance" of the FET times the average inductor
current.
FET on resistance increases at VIN values less than 5 V, since the internal N-FET has less gate voltage in this
input voltage range (see Typical Characteristics curves). Above VIN = 5V, the FET gate voltage is internally
clamped to 5V.
The maximum peak switch current the device can deliver is dependent on duty cycle. For higher duty cycles, see
Typical Characteristics.
8.2.2.13 Inductor Suppliers
Recommended suppliers of inductors for this product include, but are not limited to Sumida, Coilcraft, Panasonic,
TDK, and Murata. When selecting an inductor, make certain that the continuous current rating is high enough to
avoid saturation at peak currents. A suitable core type must be used to minimize core (switching) losses, and
wire power losses must be considered when selecting the current rating.
Copyright © 2004–2015, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LM2731
EFFICIENCY (%)
LOAD CURRENT (mA)
3.3 -5V Boost
^z_sŒ•]}v
0200 400 600 800
70
80
90
100
LM2731 ³<´
SW
FB
GND
VIN
SHDN
U1
R3
51K
SHDN
GND
3.3 VIN
C1
2.2PFR2
13.3K
CF
470pF
D1
MBR0520
R1/40.5K
L1/6.8PH
C2
22PF
5V
OUT
700mA
(TYP)
0 20 40 60 80 100 120 140 160
LOAD (mA)
0
10
20
30
40
50
60
70
80
EFFICIENCY (%)
700
0100 200 300 400 500 600
LOAD (mA)
EFFICIENCY (%)
0
10
20
30
40
50
60
70
80
90
100
LM2731
SNVS217G –MAY 2004–REVISED SEPTEMBER 2015
www.ti.com
8.2.3 Application Curves
See Typical Characteristics.
VIN = 3.3 V VOUT = 5 V VIN = 3.3 V VOUT = 12 V
Figure 32. Efficiency vs Load Current - X Option Figure 33. Efficiency vs Load Current - X Option
8.3 System Examples
Figure 34. VIN = 3.3 V, VOUT = 5 V at 700 mA Figure 35. Efficiency vs Load Current
18 Submit Documentation Feedback Copyright © 2004–2015, Texas Instruments Incorporated
Product Folder Links: LM2731
LM2731"Y"
L1 / 1.5 PH
R3
51K
C1
4.7PFC2
4.7PF
R2
120
D1
MBR0520
WHITE
LED's
VIN SW
FB
GND
SHDN
B1
LI-ION
3.3 - 4.2V
FLASH
ENABLE
0
-+
EFFICIENCY (%)
LOAD (mA)
050 100 150 200 250
0
10
20
30
40
50
60
70
80
90
100
300
3.3 -9V
^y_sŒ•]}v
LM2731 ³;´
SW
FB
GND
VIN
SHDN
U1
R3
51K
SHDN
GND
3.3 VIN
C1
2.2PFR2
13.3K
CF
330pF
D1
MBR0520
R1/84K
L1/10PH
C2
4.7PF
D2
D3
D4
D5
R4 R5
9V OUT
240mA (typ)
EFFICIENCY (%)
LOAD (mA)
0
10
20
30
40
50
60
70
80
90
100
050 100 150 200 250
3.3 -12V
Boost
^z_sŒ•]}v
LM2731 ³<´
SW
FB
GND
VIN
SHDN
U1
R3
51K
SHDN
GND
3.3 VIN
C1
2.2PFR2
13.3K
CF
270pF
D1
MBR0520
R1/117K
L1/6.8PH
C2
10PF
12V
OUT
230mA
(TYP)
LM2731
www.ti.com
SNVS217G –MAY 2004–REVISED SEPTEMBER 2015
Figure 37. Efficiency vs Load Current
Figure 36. VIN = 3.3 V, VOUT = 12 V at 230 mA
Figure 39. Efficiency vs Load Current
Figure 38. VIN = 3.3 V, VOUT = 9 V at 240 mA
Figure 40. White LED Flash Application
Copyright © 2004–2015, Texas Instruments Incorporated Submit Documentation Feedback 19
Product Folder Links: LM2731
LM2731
SNVS217G –MAY 2004–REVISED SEPTEMBER 2015
www.ti.com
9 Power Supply Recommendations
The LM2731 device is designed to operate from various DC power supplies. The impedance of the input supply
rail should be low enough that the input current transient does not cause a drop below SHUTDOWN level. If the
input supply is connected by using long wires, additional bulk capacitance may be required in addition to normal
input capacitor.
10 Layout
10.1 Layout Guidelines
High-frequency switching regulators require very careful layout of components to get stable operation and low
noise. All components must be as close as possible to the LM2731 device. TI recommends that a 4-layer PCB
be used so that internal ground planes are available.
As an example, a recommended layout of components is shown in Figure 41.
Some additional guidelines to be observed:
• Keep the path between L1, D1, and C2 extremely short. Parasitic trace inductance in series with D1 and C2
will increase noise and ringing.
• The feedback components R1, R2 and CF must be kept close to the FB pin of U1 to prevent noise injection
on the FB pin trace.
• If internal ground planes are available (recommended), use vias to connect directly to ground at pin 2 of U1,
as well as the negative sides of capacitors C1 and C2.
10.2 Layout Example
Figure 41. Recommended PCB Component Layout
20 Submit Documentation Feedback Copyright © 2004–2015, Texas Instruments Incorporated
Product Folder Links: LM2731
LM2731
www.ti.com
SNVS217G –MAY 2004–REVISED SEPTEMBER 2015
10.3 Thermal Considerations
At higher duty cycles, the increased ON-time of the FET means the maximum output current will be determined
by power dissipation within the LM2731 FET switch. The switch power dissipation from ON-state conduction is
calculated by:
P(SW) = DC × IIND(AVE)2× RDS(ON) (11)
There will be some switching losses as well, so some derating needs to be applied when calculating IC power
dissipation.
Copyright © 2004–2015, Texas Instruments Incorporated Submit Documentation Feedback 21
Product Folder Links: LM2731
LM2731
SNVS217G –MAY 2004–REVISED SEPTEMBER 2015
www.ti.com
11 Device and Documentation Support
11.1 Device Support
11.1.1 Third-Party Products Disclaimer
TI'S PUBLICATION OF INFORMATION REGARDING THIRD-PARTY PRODUCTS OR SERVICES DOES NOT
CONSTITUTE AN ENDORSEMENT REGARDING THE SUITABILITY OF SUCH PRODUCTS OR SERVICES
OR A WARRANTY, REPRESENTATION OR ENDORSEMENT OF SUCH PRODUCTS OR SERVICES, EITHER
ALONE OR IN COMBINATION WITH ANY TI PRODUCT OR SERVICE.
11.2 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
11.3 Trademarks
E2E is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
11.4 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
11.5 Glossary
SLYZ022 —TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
12 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
22 Submit Documentation Feedback Copyright © 2004–2015, Texas Instruments Incorporated
Product Folder Links: LM2731
PACKAGE OPTION ADDENDUM
www.ti.com 6-Feb-2020
Addendum-Page 1
PACKAGING INFORMATION
Orderable Device Status
(1)
Package Type Package
Drawing Pins Package
Qty Eco Plan
(2)
Lead/Ball Finish
(6)
MSL Peak Temp
(3)
Op Temp (°C) Device Marking
(4/5)
Samples
LM2731XMF NRND SOT-23 DBV 5 1000 TBD Call TI Call TI -40 to 125 S51A
LM2731XMF/NOPB ACTIVE SOT-23 DBV 5 1000 Green (RoHS
& no Sb/Br) SN Level-1-260C-UNLIM -40 to 125 S51A
LM2731XMFX/NOPB ACTIVE SOT-23 DBV 5 3000 Green (RoHS
& no Sb/Br) SN Level-1-260C-UNLIM -40 to 125 S51A
LM2731YMF ACTIVE SOT-23 DBV 5 1000 TBD Call TI Call TI -40 to 125 S51B
LM2731YMF/NOPB ACTIVE SOT-23 DBV 5 1000 Green (RoHS
& no Sb/Br) SN Level-1-260C-UNLIM -40 to 125 S51B
LM2731YMFX/NOPB ACTIVE SOT-23 DBV 5 3000 Green (RoHS
& no Sb/Br) SN Level-1-260C-UNLIM -40 to 125 S51B
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of <=1000ppm threshold. Antimony trioxide based
flame retardants must also meet the <=1000ppm threshold requirement.
(3) MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
(4) There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device.
(5) Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation
of the previous line and the two combined represent the entire Device Marking for that device.
(6) Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish
value exceeds the maximum column width.
PACKAGE OPTION ADDENDUM
www.ti.com 6-Feb-2020
Addendum-Page 2
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device Package
Type Package
Drawing Pins SPQ Reel
Diameter
(mm)
Reel
Width
W1 (mm)
A0
(mm) B0
(mm) K0
(mm) P1
(mm) W
(mm) Pin1
Quadrant
LM2731XMF SOT-23 DBV 5 1000 178.0 8.4 3.2 3.2 1.4 4.0 8.0 Q3
LM2731XMF/NOPB SOT-23 DBV 5 1000 178.0 8.4 3.2 3.2 1.4 4.0 8.0 Q3
LM2731XMFX/NOPB SOT-23 DBV 5 3000 178.0 8.4 3.2 3.2 1.4 4.0 8.0 Q3
LM2731YMF SOT-23 DBV 5 1000 178.0 8.4 3.2 3.2 1.4 4.0 8.0 Q3
LM2731YMF/NOPB SOT-23 DBV 5 1000 178.0 8.4 3.2 3.2 1.4 4.0 8.0 Q3
LM2731YMFX/NOPB SOT-23 DBV 5 3000 178.0 8.4 3.2 3.2 1.4 4.0 8.0 Q3
PACKAGE MATERIALS INFORMATION
www.ti.com 29-Sep-2019
Pack Materials-Page 1
*All dimensions are nominal
Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)
LM2731XMF SOT-23 DBV 5 1000 210.0 185.0 35.0
LM2731XMF/NOPB SOT-23 DBV 5 1000 210.0 185.0 35.0
LM2731XMFX/NOPB SOT-23 DBV 5 3000 210.0 185.0 35.0
LM2731YMF SOT-23 DBV 5 1000 210.0 185.0 35.0
LM2731YMF/NOPB SOT-23 DBV 5 1000 210.0 185.0 35.0
LM2731YMFX/NOPB SOT-23 DBV 5 3000 210.0 185.0 35.0
PACKAGE MATERIALS INFORMATION
www.ti.com 29-Sep-2019
Pack Materials-Page 2
www.ti.com
PACKAGE OUTLINE
C
0.22
0.08 TYP
0.25
3.0
2.6
2X 0.95
1.9
1.45
0.90
0.15
0.00 TYP
5X 0.5
0.3
0.6
0.3 TYP
8
0 TYP
1.9
A
3.05
2.75
B
1.75
1.45
(1.1)
SOT-23 - 1.45 mm max heightDBV0005A
SMALL OUTLINE TRANSISTOR
4214839/E 09/2019
NOTES:
1. All linear dimensions are in millimeters. Any dimensions in parenthesis are for reference only. Dimensioning and tolerancing
per ASME Y14.5M.
2. This drawing is subject to change without notice.
3. Refernce JEDEC MO-178.
4. Body dimensions do not include mold flash, protrusions, or gate burrs. Mold flash, protrusions, or gate burrs shall not
exceed 0.15 mm per side.
0.2 C A B
1
34
5
2
INDEX AREA
PIN 1
GAGE PLANE
SEATING PLANE
0.1 C
SCALE 4.000
www.ti.com
EXAMPLE BOARD LAYOUT
0.07 MAX
ARROUND 0.07 MIN
ARROUND
5X (1.1)
5X (0.6)
(2.6)
(1.9)
2X (0.95)
(R0.05) TYP
4214839/E 09/2019
SOT-23 - 1.45 mm max heightDBV0005A
SMALL OUTLINE TRANSISTOR
NOTES: (continued)
5. Publication IPC-7351 may have alternate designs.
6. Solder mask tolerances between and around signal pads can vary based on board fabrication site.
SYMM
LAND PATTERN EXAMPLE
EXPOSED METAL SHOWN
SCALE:15X
PKG
1
34
5
2
SOLDER MASK
OPENING
METAL UNDER
SOLDER MASK
SOLDER MASK
DEFINED
EXPOSED METAL
METAL
SOLDER MASK
OPENING
NON SOLDER MASK
DEFINED
(PREFERRED)
SOLDER MASK DETAILS
EXPOSED METAL
www.ti.com
EXAMPLE STENCIL DESIGN
(2.6)
(1.9)
2X(0.95)
5X (1.1)
5X (0.6)
(R0.05) TYP
SOT-23 - 1.45 mm max heightDBV0005A
SMALL OUTLINE TRANSISTOR
4214839/E 09/2019
NOTES: (continued)
7. Laser cutting apertures with trapezoidal walls and rounded corners may offer better paste release. IPC-7525 may have alternate
design recommendations.
8. Board assembly site may have different recommendations for stencil design.
SOLDER PASTE EXAMPLE
BASED ON 0.125 mm THICK STENCIL
SCALE:15X
SYMM
PKG
1
34
5
2
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