VOUT = 2.5V/2A
CIN COUT
RFB1
RFB2
L1
D1
Q1
10 PF100 PF
10 PH
1k
2.15k
Si2343
LM3475
VIN
EN
GND
PGATE
FB 1
2
3
4
5
CFF
1 nF
VIN = 5V
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Folder
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LM3475
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LM3475 Hysteretic PFET Buck Controller
1 Features 3 Description
The LM3475 is a hysteretic P-FET buck controller
1• Easy-to-Use Control Methodology designed to support a wide range of high efficiency
• 0.8 V to VIN Adjustable Output Range applications in a very small SOT-23-5 package. The
• High Efficiency (90% Typical) hysteretic control scheme has several advantages,
including simple system design with no external
• ±0.9% (±1.5% Over Temperature) Feedback compensation, stable operation with a wide range of
Voltage components, and extremely fast transient response.
• 100% Duty Cycle Capable Hysteretic control also provides high efficiency
• Maximum Operating Frequency up to 2 MHz operation, even at light loads. The PFET architecture
allows for low component count as well as 100% duty
• Internal Soft-Start cycle and ultra-low dropout operation.
• Enable Pin
Device Information(1)
2 Applications PART NUMBER PACKAGE BODY SIZE (NOM)
• TFT Monitor LM3475 SOT-23 (5) 1.60 mm × 2.90 mm
• Auto PC (1) For all available packages, see the orderable addendum at
• Vehicle Security the end of the data sheet.
• Navigation Systems
• Notebook Standby Supply
• Battery Powered Portable Applications
• Distributed Power Systems
Typical Application
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.
LM3475
SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
www.ti.com
Table of Contents
7.4 Device Functional Modes........................................ 10
1 Features.................................................................. 18 Application and Implementation ........................ 11
2 Applications ........................................................... 18.1 Application Information............................................ 11
3 Description............................................................. 18.2 Typical Application ................................................. 11
4 Revision History..................................................... 29 Power Supply Recommendations...................... 16
5 Pin Configuration and Functions......................... 310 Layout................................................................... 16
6 Specifications......................................................... 410.1 Layout Guidelines ................................................. 16
6.1 Absolute Maximum Ratings ...................................... 410.2 Layout Example .................................................... 17
6.2 ESD Ratings.............................................................. 411 Device and Documentation Support................. 18
6.3 Recommended Operating Ratings ........................... 411.1 Device Support...................................................... 18
6.4 Thermal Information.................................................. 411.2 Community Resources.......................................... 18
6.5 Electrical Characteristics........................................... 511.3 Trademarks........................................................... 18
6.6 Typical Characteristics.............................................. 611.4 Electrostatic Discharge Caution............................ 18
7 Detailed Description.............................................. 811.5 Glossary................................................................ 18
7.1 Overview................................................................... 812 Mechanical, Packaging, and Orderable
7.2 Functional Block Diagram......................................... 8Information........................................................... 18
7.3 Feature Description................................................... 8
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision B (March 2013) to Revision C 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
Changes from Revision A (March 2013) to Revision B Page
• Changed layout of National Data Sheet to TI format. ........................................................................................................... 1
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PGATE
GND
FB
VIN
1
2
34
5
EN
LM3475
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SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
5 Pin Configuration and Functions
DBV Package
5-Pin SOT-23
Top View
Pin Functions
PIN I/O DESCRIPTION
NAME NO.
FB 1 I Feedback input. Connect to a resistor divider between the output and GND.
GND 2 G Ground.
Enable. Pull this pin above 1.5 V (typical) for normal operation. When EN is low, the device
EN 3 O enters shutdown mode.
VIN 4 P Power supply input.
PGATE 5 O Gate drive output for the external PFET.
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6 Specifications
6.1 Absolute Maximum Ratings
See (1)(2)
MIN MAX UNIT
VIN −0.3 16 V
PGATE −0.3 16 V
FB −0.3 5 V
EN −0.3 16 V
Power dissipation (3) 440 mW
Vapor phase (60 s) 215
Lead temperature °C
Infrared (15 s) 220
Tstg Storage temperature −65 1150 °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) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
(3) The maximum allowable power dissipation is a function of the maximum junction temperature, TJ_MAX, the junction-to-ambient thermal
resistance, θJA and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated
using: PD_MAX = (TJ_MAX - TA)/θJA. The maximum power dissipation of 0.44 W is determined using TA= 25°C, θJA = 225°C/W, and
TJ_MAX = 125°C.
6.2 ESD Ratings
over operating free-air temperature range (unless otherwise noted) VALUE UNIT
V(ESD) Electrostatic discharge Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001(1) 2500 V
(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
6.3 Recommended Operating Ratings
MIN NOM MAX UNIT
Supply voltage 2.7 10 V
TJOperating junction temperature -40 125 °C
6.4 Thermal Information LM3475
THERMAL METRIC(1) DBV (SOT-23) UNIT
5 PINS
RθJA Junction-to-ambient thermal resistance 164.2 °C/W
RθJC(top) Junction-to-case (top) thermal resistance 115.3 °C/W
RθJB Junction-to-board thermal resistance 27.0 °C/W
ψJT Junction-to-top characterization parameter 12.8 °C/W
ψJB Junction-to-board characterization parameter 26.5 °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
Typical limits are for TJ= 25°C, unless otherwise specified, VIN = EN = 5.0 V. Maximum and minimum specification limits are
specified by design, test, or statistical analysis.
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
IQQuiescent current EN = VIN (PGATE TJ= 25°C 260
Open) TJ=−40°C to +125°C 170 320 µA
EN = 0V TJ= 25°C 7
TJ=−40°C to +125°C 4 10
VFB TJ= 25°C 0.8
Feedback voltage V
TJ=−40°C to +125°C 0.788 0.812
%ΔVFB/ΔVIN Feedback voltage line 2.7 V < VIN < 10 V 0.01 %/V
regulation
VHYST Comparator hysteresis 2.7 V < VIN < 10 V TJ= 25°C 21 28 mV
−40°C to +125°C 21 32
IFB FB bias current TJ= 25°C 50 nA
−40°C to +125°C 600
Enable threshold Increasing TJ= 25°C 1.5 V
voltage
VthEN −40°C to +125°C 1.2 1.8
Hysteresis 365 mV
IEN Enable leakage current TJ= 25°C 0.025
EN = 10 V µA
−40°C to +125°C 1
Source 2.8
ISOURCE = 100 mA
RPGATE Driver resistance Ω
Sink 1.8
ISink = 100 mA
Source
VPGATE = 3.5 V 0.475
CPGATE = 1 nF
IPGATE Driver output current A
Sink
VPGATE = 3.5 V 1.0
CPGATE = 1 nF
TSS Soft-start time 2.7 V < VIN < 10 V (EN Rising) 4 ms
TONMIN Minimum on-time PGATE Open 180 ns
VUVD Undervoltage detection Measured at the FB TJ= 25°C 0.56 V
Pin −40°C to +125°C 0.487 0.613
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45 6 7 8 9 10
INPUT VOLTAGE (VIN)
0
20
40
60
80
100
EFFICIENCY (%)
84
86
88
90
92
94
96
98
EFFICIENCY (%)
OUTPUT CURRENT (A)
0.5 1 1.5 2
LM3475
SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
www.ti.com
6.6 Typical Characteristics
Unless specified otherwise, all curves taken at VIN = 5 V, VOUT = 2.5 V, L = 10 µH, COUT = 100 µF, ESR = 100 mΩ, and TA=
25°C.
Figure 1. Quiescent Current vs Input Voltage Figure 2. Feedback Voltage vs Temperature
Figure 3. Hysteresis Voltage vs Input Voltage Figure 4. Hysteresis Voltage vs Temperature
IOUT = 2 A
Figure 5. Efficiency vs Load Current Figure 6. Efficiency vs Input Voltage
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VSW
2.5V/DIV
VRIPPLE
20 mV/DIV
2 Ps/DIV
VIN
2V/DIV
VOUT
1V/DIV
1 ms/DIV
LM3475
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SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
Typical Characteristics (continued)
Unless specified otherwise, all curves taken at VIN = 5 V, VOUT = 2.5 V, L = 10 µH, COUT = 100 µF, ESR = 100 mΩ, and TA=
25°C.
Figure 7. Start Up Figure 8. Output Ripple Voltage
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PGATE
GND
Internal
Regulator
VCC
Level
Shift en
Band Gap
Reference VREF
70% VREF
Soft-Start
Vref Ramp
+
-
+
-
Current
Bias reset
Blanking
Timer
UVD-Disable
UVD-Disable
en
reset
FB
EN
VIN Pdrive
UVD
Comp
Hysteretic
Comp
LM3475
SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
www.ti.com
7 Detailed Description
7.1 Overview
The LM3475 is a buck (step-down) DC-DC controller that uses a hysteretic control architecture, which results in
Pulse Frequency Modulated (PFM) regulation. The hysteretic control scheme does not utilize an internal
oscillator. Switching frequency depends on external components and operating conditions. Operating frequency
decreases at light loads, resulting in excellent efficiency compared to PWM architectures. Because switching is
directly controlled by the output conditions, hysteretic control provides exceptional load transient response.
7.2 Functional Block Diagram
7.3 Feature Description
7.3.1 Hysteretic Control Circuit
The LM3475 uses a comparator-based voltage control loop. The voltage on the feedback pin is compared to a
0.8V reference with 21mV of hysteresis. When the FB input to the comparator falls below the reference voltage,
the output of the comparator goes low. This results in the driver output, PGATE, pulling the gate of the PFET low
and turning on the PFET.
With the PFET on, the input supply charges COUT and supplies current to the load through the PFET and the
inductor. Current through the inductor ramps up linearly, and the output voltage increases. As the FB voltage
reaches the upper threshold (reference voltage plus hysteresis) the output of the comparator goes high, and the
PGATE turns the PFET off. When the PFET turns off, the catch diode turns on, and the current through the
inductor ramps down. As the output voltage falls below the reference voltage, the cycle repeats. The resulting
output, inductor current, and switch node waveforms are shown in Figure 9.
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td
VOUT
(DC) VHYST
VOUT
ripple
VIN
-VD
tON
Switch Voltage
Iout 'IL
Inductor Current
Output Voltage
tOFF
td
LM3475
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SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
Feature Description (continued)
Figure 9. Hysteretic Waveforms
7.3.2 Soft-Start
The LM3475 includes an internal soft-start function to protect components from excessive inrush current and
output voltage overshoot. As VIN rises above 2.7 V (typical), the internal bias circuitry becomes active. When EN
goes high, the device enters soft-start. During soft-start, the reference voltage is ramped up to the nominal value
of 0.8 V in approximately 4ms. Duty cycle and output voltage will increase as the reference voltage is ramped up.
7.3.3 Under Voltage Detection
When the output voltage falls below 70% (typical) of the normal voltage, as measured at the FB pin, the device
turns off PFET and restarts a new soft-start cycle. In short circuit, the PFET is always on, and the converter is
effectively a resistor divider from input to output to ground. Whether the part restarts depends on the power path
resistance and the short circuit resistance. This feature should not be considered as overcurrent protection or
output short circuit protection.
7.3.4 PGATE
During switching, the PGATE pin swings from VIN (off) to ground (on). As input voltage increases, the time it
takes to slew the gate of the PFET on and off also increases. Also, as the PFET gate voltage approaches VIN,
the PGATE current driving capability decreases. This can cause a significant additional delay in turning the
switch off when using a PFET with a low threshold voltage. These two effects will increase power dissipation and
reduce efficiency. Therefore, a PFET with relatively high threshold voltage and low gate capacitance is
recommended.
7.3.5 Minimum On or Off Time
To ensure accurate comparator switching, the LM3475 imposes a blanking time after each comparator state
change. This blanking time is 180 ns typically. Immediately after the comparator goes high or low, it will be held
in that state for the duration of the blanking time. This helps keep the hysteretic comparator from improperly
responding to switching noise spikes (See Reducing Switching Noise) and ESL spikes (See Output Capacitor
Selection) at the output.
At very low or very high duty cycle operation, maximum frequency will be limited by the blanking time. The
maximum operating frequency can be determined by the following equations:
FMAX = D / tonmin (1)
FMAX = (1-D) / toffmin
where
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Feature Description (continued)
• D is the duty cycle, defined as VOUT/VIN, and tonmin
• toffmin is the sum of the blanking time, the propagation delay time, and the PFET delay time (see Figure 9) (2)
7.3.6 Enable Pin (EN)
The LM3475 provides a shutdown function via the EN pin to disable the device. The device is active when the
EN pin is pulled above 1.5 V (typ) and in shutdown mode when EN is below 1.135 V (typ). In shutdown mode,
total quiescent current is less than 10 µA. The EN pin can be directly connected to VIN for always-on operation.
7.4 Device Functional Modes
The LM3475 operates in discontinuous conduction mode at light load current and continuous conduction mode at
heavy load current. In discontinuous conduction mode, current through the inductor starts at zero and ramps up
to the peak, then ramps down to zero. The next cycle starts when the FB voltage reaches the reference voltage.
Until then, the inductor current remains zero. Operating frequency is low, as are switching losses. In continuous
conduction mode, current always flows through the inductor and never ramps down to zero.
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VOUT = 2.5V/2A
CIN COUT
RFB1
RFB2
L1
D1
Q1
10 PF100 PF
10 PH
1k
2.15k
Si2343
LM3475
VIN
EN
GND
PGATE
FB 1
2
3
4
5
CFF
1 nF
VIN = 5V
LM3475
www.ti.com
SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
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 LM3475 employs a hysteretic control architecture; which provides excellent load transient response and
efficiency even at light loads, as compared to its PWM architectures. No external compensation is required which
results in a simple design and low component count. A typical schematic is described in the next section.
8.2 Typical Application
Figure 10. Full Demo Board Schematic
8.2.1 Design Requirements
To properly size the components for the application, the designer needs the following parameters: input voltage
range, output voltage, output current range, and required switching frequency. These four main parameters affect
the choices of component available to achieve a proper system behavior. Although hysteretic control is a simple
control scheme, the operating frequency and other performance characteristics depend on external conditions
and components. If the inductance, output capacitance, ESR, VIN, or Cff is changed, there will be a change in
the operating frequency and possibly output ripple. Therefore, care must be taken to select components which
will provide the desired operating range.
8.2.2 Detailed Design Procedure
Table 1. Bill of Materials
DESIGNATOR DESCRIPTION PART NUMBER VENDOR
CIN 10 µF, 16 V, X5R EMK325BJ106MN TAIYO YUDEN
COUT 100 µF, 6 V, Ta TPSY107M006R0100 AVX
CFF 1 nF, 25 V, X7R VJ1206Y102KXXA Vishay
D1 Schottky, 20 V, 2 A CMSH2-20L Central Semiconductor
L1 10 µH, 3.1 A CDRH103R100 Sumida
Q1 30 V, 2.5 A Si2343 Vishay
RFB2 1 kΩ, 0805, 1% CRW08051001F Vishay
RFB1 2.15 kΩ, 0805, 1% CRCW08052151F Vishay
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F = (VIN - VOUT) x ESR
x(VHYST x D x L) + (VIN x delay x ESR)
VIN
VOUT
-
+
PMOS_drv
Hyst Comp
FB
+
VOUT
PGATE
+
-
Reference
Voltage
VFB = 0.8V
VHYST = 21 mV
PGATE
R1
R2
L
Cff COUT
VOUT = R1 + R2
R2x VFB
LM3475
SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
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8.2.2.1 Setting Output Voltage
The output voltage is programmed using a resistor divider between VOUT and GND as shown in Figure 11. The
feedback resistors can be calculated as follows:
where
• Vfb is 0.8 V typically (3)
The feedback resistor ratio, α= (R1+R2) / R2, will also be used below to calculate output ripple and operating
frequency.
Figure 11. Hysteretic Window
8.2.2.2 Setting Operating Frequency and Output Ripple
Although hysteretic control is a simple control scheme, the operating frequency and other performance
characteristics depend on external conditions and components. If the inductance, output capacitance, ESR, VIN,
or Cff is changed, there will be a change in the operating frequency and possibly output ripple. Therefore, care
must be taken to select components which will provide the desired operating range. The best approach is to
determine what operating frequency is desirable in the application and then begin with the selection of the
inductor and output capacitor ESR. The design process usually involves a few iterations to select appropriate
standard values that will result in the desired frequency and ripple.
Without the feedforward capacitor (Cff), the operating frequency (F) can be approximately calculated using the
formula:
where
• Delay is the sum of the LM3475 propagation delay time and the PFET delay time
• The propagation delay is 90ns typically (4)
Minimum output ripple voltage can be determined using the following equation:
VOUT_PP = VHYST ( R1 + R2 ) / R2 (5)
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F = VIN
2S x R1 x C2 x VHYS
'I2
3
IOUT2 +
IRMS =
L = VIN - VSD - VOUT
'IxF
D
LM3475
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SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
8.2.2.3 Using a Feed-forward Capacitor
The operating frequency and output ripple voltage can also be significantly influenced using a speed up
capacitor, Cff, as shown in Figure 11. Cff is connected in parallel with the high side feedback resistor, R1. The
output ripple causes a current to be sourced or sunk through this capacitor. This current is essentially a square
wave. Since the input to the feedback pin (FB) is a high impedance node, the bulk of the current flows through
R2. This superimposes a square wave ripple voltage on the FB node. The end result is a reduction in output
ripple and an increase in operating frequency. When adding Cff, calculate the formula above with α= 1. The value
of Cff depends on the desired operating frequency and the value of R2. A good starting point is 1nF ceramic at
100kHz decreasing linearly with increased operating frequency. Also note that as the output voltage is
programmed below 1.6V, the effect of Cff will decrease significantly.
8.2.2.4 Inductor Selection
The most important parameters for the inductor are the inductance and the current rating. The LM3475 operates
over a wide frequency range and can use a wide range of inductance values. Minimum inductance can be
calculated using the following equation:
where
• D is the duty cycle, defined as VOUT/VIN
•ΔI is the allowable inductor ripple current (6)
Maximum allowable inductor ripple current should be calculated as a function of output current (IOUT) as shown
below:
ΔImax = IOUT x 0.3
The inductor must also be rated to handle the peak current (IPK) and RMS current given by:
IPK = (IOUT +ΔI/2) x 1.1 (7)
(8)
The inductance value and the resulting ripple is one of the key parameters controlling operating frequency.
8.2.2.5 Output Capacitor Selection
Once the desired operating frequency and inductance value are selected, ESR must be selected based on
Equation 4. This process may involve a few iterations to select standard ESR and inductance values.
In general, the ESR of the output capacitor and the inductor ripple current create the output ripple of the
regulator. However, the comparator hysteresis sets the first order value of this ripple. Therefore, as ESR and
ripple current vary, operating frequency must also vary to keep the output ripple voltage regulated. The hysteretic
control topology is well suited to using ceramic output capacitors. However, ceramic capacitors have a very low
ESR, resulting in a 90° phase shift of the output voltage ripple. This results in low operating frequency and
increased output ripple. To fix this problem a low value resistor could be added in series with the ceramic output
capacitor. Although counter intuitive, this combination of a ceramic capacitor and external series resistance
provide highly accurate control over the output voltage ripple. Another method is to add an external ramp at the
FB pin as shown in Figure 12. By proper selection of R1 and C2, the FB pin sees faster voltage change than the
output ripple can cause. As a result, the switching frequency is higher while the output ripple becomes lower. The
switching frequency is approximately:
(9)
Other types of capacitor, such as Sanyo POSCAP, OS-CON, and Nichicon ’NA’ series are also recommended
and may be used without additional series resistance. For all practical purposes, any type of output capacitor
may be used with proper circuit verification.
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VIN VOUT = 0.9V/2A
CIN COUT
RFB1
RFB2
L1
D1
Q1
10 PF100 PF
10 PH
10k
1.27k
Si2343
LM3475
VIN
EN
GND
PGATE
FB 1
2
3
4
5
C2
390 pF
C1
3.9 nF
R1
200k
IRMS_CIN = VIN
IOUT VOUT x (VIN - VOUT)
LM3475
SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
www.ti.com
Capacitors with high ESL (equivalent series inductance) values should not be used. As shown in Figure 9, the
output ripple voltage contains a small step at both the high and low peaks. This step is caused by and is directly
proportional to the output capacitor’s ESL. A large ESL, such as in an electrolytic capacitor, can create a step
large enough to cause abnormal switching behavior.
8.2.2.6 Input Capacitor Selection
A bypass capacitor is required between VIN and ground. It must be placed near the source of the external PFET.
The input capacitor prevents large voltage transients at the input and provides the instantaneous current when
the PFET turns on. The important parameters for the input capacitor are the voltage rating and the RMS current
rating. Follow the manufacturer’s recommended voltage de-rating. RMS current and power dissipation (PD) can
be calculated with the equations below:
(10)
Figure 12. External Ramp
8.2.2.7 Diode Selection
The catch diode provides the current path to the load during the PFET off time. Therefore, the current rating of
the diode must be higher than the average current through the diode, which be calculated as shown:
ID_AVE = IOUT x (1 −D) (11)
The peak voltage across the catch diode is approximately equal to the input voltage. Therefore, the diode’s peak
reverse voltage rating should be greater than 1.3 times the input voltage.
A Schottky diode is recommended, since a low forward voltage drop will improve efficiency.
For high temperature applications, diode leakage current may become significant and require a higher reverse
voltage rating to achieve acceptable performance.
8.2.2.8 P-Channel MOSFET Selection
The PFET switch should be selected based on the maximum Drain-Source voltage (VDS), Drain current rating
(ID), maximum Gate-Source voltage (VGS), on resistance (RDSON), and Gate capacitance. The voltage across the
PFET when it is turned off is equal to the sum of the input voltage and the diode forward voltage. The VDS must
be selected to provide some margin beyond the sum of the input voltage and Vd.
Since the current flowing through the PFET is equal to the current through the inductor, IDmust be rated higher
than the maximum IPK. During switching, PGATE swings the PFET’s gate from VIN to ground. Therefore, A PFET
must be selected with a maximum VGS larger than VIN. To insure that the PFET turns on completely and quickly,
refer to the PGATE section.
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5
PGATE
RPGATE
3.3:
D1
CSNUB
RSNUB
L1
Q1
LM3475
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SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
The power loss in the PFET consists of switching losses and conducting losses. Although switching losses are
difficult to precisely calculate, the equation below can be used to estimate total power dissipation. Increasing
RDSON will increase power losses and degrade efficiency. Note that switching losses will also increase with lower
gate threshold voltages.
PDswitch = RDSONx (IOUT)2x D + F x IOUTx VINx (ton + toff)/2
where
• ton = FET turn on time
• toff = FET turn off time
• A value of 10ns to 50ns is typical for ton and toff (12)
Note that the RDSON has a positive temperature coefficient. At 100°C, the RDSON may be as much as 150% higher
than the value at 25°C.
The Gate capacitance of the PFET has a direct impact on both PFET transition time and the power dissipation in
the LM3475. Most of the power dissipated in the LM3475 is used to drive the PFET switch. This power can be
calculated as follows:
The amount of average gate driver current required during switching (IG) is:
IG= Qgx F (13)
And the total power dissipated in the device is:
IqVIN + IGVIN
where
• Iqis typically 260µA as shown in Electrical Characteristics (14)
As gate capacitance increases, operating frequency may need to be reduced, or additional heat sinking may be
required to lower the power dissipation in the device.
In general, keeping the gate capacitance below 2000 pF is recommended to keep transition times (switching
losses), and power losses low.
8.2.2.9 Reducing Switching Noise
Although the LM3475 employs internal noise suppression circuitry, external noise may continue to be excessive.
There are several methods available to reduce noise and EMI.
MOSFETs are very fast switching devices. The fast increase in PFET current coupled with parasitic trace
inductance can create unwanted noise spikes at both the switch node and at VIN. Switching noise will increase
with load current and input voltage. This noise can also propagate through the ground plane, sometimes causing
unpredictable device performance. Slowing the rise and fall times of the PFET can be very effective in reducing
this noise. Referring to Figure 13, the PFET can be slowed down by placing a small (1-Ωto 10-Ω) resistor in
series with PGATE. However, this resistor will increase the switching losses in the PFET and will lower efficiency.
Therefore it should be kept as small as possible and only used when necessary. Another method to reduce
switching noise (other than good PCB layout, see Layout) is to use a small RC filter or snubber. The snubber
should be placed in parallel with the catch diode, connected close to the drain of the PFET, as shown in
Figure 13. Again, the snubber should be kept as small as possible to limit its impact on system efficiency. A
typical range is a 10-Ωto 100-Ωresistor and a 470-pF to 2.2-nF ceramic capacitor.
Figure 13. PGATE Resistor and Snubber
Copyright © 2004–2015, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LM3475
LM3475
SNVS239C –OCTOBER 2004–REVISED OCTOBER 2015
www.ti.com
8.2.3 Application Curves
Figure 14. Load Transient Response with External Ramp Figure 15. Load Transient Response
(Circuit from Figure 12) (Typical Application Circuit from Figure 16)
9 Power Supply Recommendations
The LM3475 controller is designed to operate from various DC power supplies. VIN input should be protected
from reversal voltage and voltage dump over 16 volts. The impedance of the input supply rail should be low
enough that the input current transient does not cause drop below VIN UVLO 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
PC board layout is very important in all switching regulator designs. Poor layout can cause EMI problems, excess
switching noise and poor operation.
As shown in Figure 16, place the ground of the input capacitor as close as possible to the anode of the diode.
This path also carries a large AC current. The switch node, the node connecting the diode cathode, inductor, and
PFET drain, should be kept as small as possible. This node is one of the main sources for radiated EMI.
The feedback pin is a high impedance node and is therefore sensitive to noise. Be sure to keep all feedback
traces away from the inductor and the switch node, which are sources of noise. Also, the resistor divider should
be placed close to the FB pin. The gate pin of the external PFET should be located close to the PGATE pin.
TI also recommends using a large, continuous ground plane, particularly in higher current applications.
16 Submit Documentation Feedback Copyright © 2004–2015, Texas Instruments Incorporated
Product Folder Links: LM3475
LM3475
SNVS239C –OCTOBER 2004–REVISED OCTOBER 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.
18 Submit Documentation Feedback Copyright © 2004–2015, Texas Instruments Incorporated
Product Folder Links: LM3475
PACKAGE OPTION ADDENDUM
www.ti.com 10-Dec-2020
Addendum-Page 1
PACKAGING INFORMATION
Orderable Device Status
(1)
Package Type Package
Drawing Pins Package
Qty Eco Plan
(2)
Lead finish/
Ball material
(6)
MSL Peak Temp
(3)
Op Temp (°C) Device Marking
(4/5)
Samples
LM3475MF/NOPB ACTIVE SOT-23 DBV 5 1000 RoHS & Green SN Level-1-260C-UNLIM -40 to 125 S65B
LM3475MFX/NOPB ACTIVE SOT-23 DBV 5 3000 RoHS & Green SN Level-1-260C-UNLIM -40 to 125 S65B
(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 finish/Ball material - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead finish/Ball material values may wrap to two
lines if the finish value exceeds the maximum column width.
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.
PACKAGE OPTION ADDENDUM
www.ti.com 10-Dec-2020
Addendum-Page 2
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
LM3475MF/NOPB SOT-23 DBV 5 1000 178.0 8.4 3.2 3.2 1.4 4.0 8.0 Q3
LM3475MFX/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 20-Dec-2016
Pack Materials-Page 1
*All dimensions are nominal
Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)
LM3475MF/NOPB SOT-23 DBV 5 1000 210.0 185.0 35.0
LM3475MFX/NOPB SOT-23 DBV 5 3000 210.0 185.0 35.0
PACKAGE MATERIALS INFORMATION
www.ti.com 20-Dec-2016
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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