LM3551/2
VC
SW
FB
FET-T
FET-F
-+
RTRF
D1
L1
RC
CC
SS
OVP
FTO
CSS
SD/
EN
T/F
LUMILED
LXCL-PWF1
Flash LED
CFTO
COUT
CIN
VBAT VIN
Sharp
GM5BW05340A
Flash LED
LM3551/2
VC
SW
FB
FET-T
FET-F
-+
RTRF
D1
L1
RC
CC
SS
OVP
FTO
CSS
SD/
EN
T/F
COUT
CFTO
CIN
GND
VIN
VBAT
LM3551, LM3552
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SNVS371D –AUGUST 2005–REVISED MAY 2013
LM3551 /LM3552 1A White LED Driver with Flash Timeout Protection
Check for Samples: LM3551,LM3552
1FEATURES DESCRIPTION
The LM3551 and LM3552 are fixed frequency,
2• Up to 1A Total Drive Current current mode step-up DC/DC converters with two
• Flash Timeout Protection integrated NFETs that can be used for precision LED
• Independent Torch/Flash/Shutdown Modes brightness control. The devices are capable of driving
loads up to 1A from a single-cell Li-Ion battery.
• LED Disconnect in Shutdown
• Programmable Soft-Start Limits Inrush Current The LM3551 and LM3552 can drive one or more high
current flash LEDs either in a high power Flash mode
• Over-Voltage Protection or a lower power Torch mode using the
• Wide Voltage Range 2.7 to 5.5V TORCH/FLASH pin. A programmable Timeout
• 1.25MHz Constant Switching Frequency function on the FTO pin forces the internal NFETs to
turn off after a certain user defined time. An external
• Small, Low Profile Package, Non-Pullback SD pin (LM3551) or EN pin (LM3552) is available to
WSON(4mm x 4mm) put the device into low power shutdown mode. During
shutdown, the feedback resistors and the load are
APPLICATIONS disconnected from the input to avoid leakage current
• White LED Camera Flash paths to ground.
• White LED Torch (Flashlight) User programmable soft-start circuitry has been
• DSC (Digital Still Camera) Flash integrated to eliminate large inrush currents at start-
up. Over-voltage protection circuitry and a 1.25MHz
• Cellular Camera Phone Flash switching frequency allow for the use of small, low-
• PDA Camera Flash cost output capacitors with lower voltage ratings. The
• Camcorder Torch (Flashlight) Lamp LM3551 and LM3552 are available in a low profile 14-
pin WSON package.
Typical Application Circuits
1Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
2All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date. Copyright © 2005–2013, Texas Instruments Incorporated
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Top View Bottom View
Die-Attach
Pad(DAP)
GND
11
10
9
12
13
8
14
4
5
6
3
2
7
1
Die-Attach
Pad(DAP)
GND
11
10
9
12
13
8
14
4
5
6
3
2
7
1
LM3551, LM3552
SNVS371D –AUGUST 2005–REVISED MAY 2013
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Connection Diagram
14-Pin WSON Package
Figure 1. Package Number NHL0014B
PIN DESCRIPTIONS
Pin Name Function
9 VIN Input Voltage. Input range: 2.7V to 5.5V.
13 T/F TORCH/FLASH Pin. Low = Torch Mode, High = Flash Mode
8 SW Switch Pin
10 OVP Over Voltage Protection Pin
3 VCCompensation network connection. Connected to the output of the voltage error amplifier.
SD(LM3551) Shutdown pin logic input. High = Shutdown, Low = Enabled
5EN(LM3552) Enable pin logic input. High = Enabled, Low = Shutdown
12 FTO Flash Timeout. External capacitor determines max. duration allowed flash pulse
11 SS Soft Start Pin
4 FB Feedback Pin
14 FET-T Torch FET Drain
2 FET-F Flash FET Drain
1,7,DAP GND Ground
Analog Ground. Connect the ground of the compensation components, CFTO and soft start cap to
6 AGND AGND. AGND must be connected to the GND pin through a low impedance connection.
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.
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Absolute Maximum Ratings(1)(2)(3)
VIN pin: Voltage to GND 7.5V
SW pin: Voltage to GND 21V
FB pin: Voltage to GND 7V
VCpin: Voltage to GND 1.26V ± 0.3V
SD,T/F pins: Voltage to GND 7.5V
FET-T, FET-F: Voltage to GND 6V
Continuous Power Dissipation(4) Internally Limited
Junction Temperature (TJ-MAX ) 150°C
Storage Temperature Range -65°C to +150
ESD Rating(5) Human Body Model 2.0kV
(1) Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under
which operation of the device is specified. Operating Ratings do not imply performance limits. For performance limits and associated test
conditions, see the Electrical Characteristics tables.
(2) All voltages are with respect to the potential at the GND pin.
(3) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
(4) Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=140ºC (typ.) and
disengages at TJ=120ºC (typ.).
(5) The human body model is a 100pF capacitor discharged through a 1.5kΩresistor into each pin. (MIL-STD-883 3015.7)
Operating Ratings(1)(2)
Input Voltage Range 2.7V to 5.5V
SW Voltage Max.(3) 20V
Junction Temperature (TJ) Range -40°C to +110°C
Ambient Temperature (TA) Range(4) -40°C to +85°C
(1) Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under
which operation of the device is specified. Operating Ratings do not imply performance limits. For performance limits and associated test
conditions, see the Electrical Characteristics tables.
(2) All voltages are with respect to the potential at the GND pin.
(3) Maximum recommended SW pin voltage when the OVP pin is grounded.
(4) In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may
have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP =
110ºC), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to-ambient thermal resistance of the
part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX).
Thermal Properties
Junction-to-Ambient Thermal Resistance (θJA), NHL0014B Package(1) 37.3°C/W
(1) Junction-to-ambient thermal resistance is highly application and board-layout dependent. In applications where high maximum power
dissipation exists, special care must be paid to thermal dissipation issues in board design.
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Electrical Characteristics(1)(2)
Limits in standard typeface are for TJ= +25° C. Limits in boldface type apply over the full operating junction temperature
range (-40°C ≤TJ≤+110°C). Unless otherwise noted, specifications apply to the LM3551 and LM3552 Typical Application
Circuit (pg. 1) with: VIN = 3.6V, V(SD) = 0V for LM3551 and V(EN) = VIN for LM3552, ILOAD = 0A(3)
Symbol Parameter Conditions Min Typ Max Units
IQQuiescent Current FB = VIN (Not Switching) 1.47 2.0 mA
V(SD) = VIN, LM3551 2.55 5.0
ISD Shutdown Current µA
V(EN) = 0V, LM3552 0.1 2.3
ICL(4) Switch Current Limit VIN = 3.0V(5) 2.1 A
VFB Feedback Voltage 1.2285 1.265 1.2915 V
IFB(6) Feedback Pin Bias Current 50 nA
gmError Amp Transconductance ΔI = 5µA 135 µmho
AVError Amp Voltage Gain 135 V/V
DMAX Maximum Duty Cycle 92.5 %
fsw Switching Frequency 0.9 1.25 1.6 MHz
Shutdown Pin Current
ISDPIN VSD = 0V 3.0 6µA
(LM3551)
Enable Pin Current
IENPIN VEN = 3.6V 3.0 6µA
(LM3552) VT/F = 0V 26
IT/FPIN T/F Pin Current nA
VT/F = VIN 22
IL-SW SW Pin Leakage Current VL-SW = 20V 0.07 8µA
RDSON-SW SW Pin RDSON ISW = 0.5A 0.165 Ω
IL-T FET-T Leakage Current 0.1 µA
RDSON-T FET-T RDSON 0.98 Ω
IL-F FET-F Leakage Current 0.1 µA
RDSON-F FET-F RDSON 0.36 Ω
Output High 1.2
Shutdown/Enable Pin
ThSD/EN V
Threshold Output Low 0.3
Output High 1.2
ThT/F T/F Pin Threshold V
Output Low 0.3
On Threshold 2.25 2.48 2.70
Under Voltage Protection
UVP V
Thresholds Off Threshold 2.43 2.58 2.77
On Threshold 11.3 12.4 14
Over Voltage Protection
OVP V
Thresholds Off Threshold 9.2 10.6 12
VFTO Flash Timeout trip-point 0.99 1.16 1.32 V
IFTO Flash Timeout Current 1.12 1.4 1.68 µA
VSS Soft-Start Voltage 1.18 1.25 1.32 V
ISS Soft-Start Current 10 11.5 13 µA
(1) All voltages are with respect to the potential at the GND pin.
(2) Min and Max limits are specified by design, test, or statistical analysis. Typical (Typ) numbers represent the most likely norm. Unless
otherwise specified, conditions for Typ specifications are: VIN = 3.6V and TA= 25ºC.
(3) CIN and COUT,: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics
(4) Duty cycle affects current limit due to ramp generator.
(5) Current limit at 0% duty cycle. See TYPICAL PERFORMANCE section for Switch Current Limit vs. VIN
(6) Bias current flows into FB pin.
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Typical Performance Characteristics
Unless otherwise specified: TA= +25°C; VIN = 3.6V; L = 4.7µH, (RC= 10kΩ, CC= 4.7nF, CIN = COUT = 10µF for Lumiled LED),
(RC= 27kΩ, CC= 10nF, CIN = 10µF, COUT = 4.7µF for Sharp LED), CFTO = 1µF, CSS= 0.1µF.
ICL measure when VOUT = 95% × VOUT (nominal)
Current Limit vs. Input Voltage Current Limit vs. Input Voltage
VOUT = 5V VOUT = 10V
Figure 2. Figure 3.
ICL measure when VOUT = 95% × VOUT (nominal)
Converter Efficiency vs. Input Voltage Converter Efficiency vs. Input Voltage
Lumiled Flash LED Sharp Flash LED
Figure 4. Figure 5.
IOUT measured at 95%× VOUT (nominal)
Maximum IOUT vs. Input Voltage Maximum IOUT vs. Input Voltage
VOUT = 5V VOUT = 10V
Figure 6. Figure 7.
IOUT measured at 95%× VOUT (nominal)
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Typical Performance Characteristics (continued)
Unless otherwise specified: TA= +25°C; VIN = 3.6V; L = 4.7µH, (RC= 10kΩ, CC= 4.7nF, CIN = COUT = 10µF for Lumiled LED),
(RC= 27kΩ, CC= 10nF, CIN = 10µF, COUT = 4.7µF for Sharp LED), CFTO = 1µF, CSS= 0.1µF.
LED Torch Current LED Flash Current
vs. vs.
Input Voltage Input Voltage
Lumiled Flash LED Lumiled Flash LED
Figure 8. Figure 9.
OVP Trip Voltage Switching Frequency
vs. vs.
Input Voltage Input Voltage
Figure 10. Figure 11.
Start-Up Waveform Start-Up Waveform
Sharp LED Lumiled LED
Figure 12. Ch1 = VSD, Ch3 = ILED, Ch4 = IIN Figure 13. Ch1 = VSD, Ch3 = ILED, Ch4 = IIN
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+
-
+
-
+
-
LOGIC
Drive
UVP
Reset
OVP
Set Reset
Thermal SD
¦
Oscillator
Duty
Cycle
Limit
Load Current
Measurement
+
-
Thermal
Shutdown
Shutdown
Comparator OVP
Flash
Timeout
Bandgap
Voltage
Reference
BG
SW
SS
VIN GNDFTO
VC
FB
OVP
FET-F
FET-T SD / EN
OVP COMP
PWM COMP
ERROR AMP
GND AGND
LM3551/2
VIN
SD/EN
LM3551
LM3552
T/F
LM3551, LM3552
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SNVS371D –AUGUST 2005–REVISED MAY 2013
Typical Performance Characteristics (continued)
Unless otherwise specified: TA= +25°C; VIN = 3.6V; L = 4.7µH, (RC= 10kΩ, CC= 4.7nF, CIN = COUT = 10µF for Lumiled LED),
(RC= 27kΩ, CC= 10nF, CIN = 10µF, COUT = 4.7µF for Sharp LED), CFTO = 1µF, CSS= 0.1µF.
Typical Switching Waveform
Figure 14. Sharp LED in Flash Mode
Ch1 = VSW, Ch3 = ILED, Ch4 = IL
Block Diagram
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Circuit Description
OVERVIEW
The LM3551/2 are high power white LED flash drivers capable of delivering up to 1A of output current. The
devices utilize a highly efficient inductive DC/DC boost converter to achieve the required output voltage. A
current-mode PWM control scheme regulates the output current over a wide input voltage range. Both the
LM3551 and the LM3552 have two low-side load disconnect FET's allowing for a continuous low-power Torch-
mode and a high-power, short duration Flash-mode.
Several application specific safety features are integrated into the LM3551/2 design. A flash timeout circuit is
present on-chip to prevent a failure in the flash LED caused by a timing violation. Over-Voltage Protection
protects the output capacitor, inductor and main power switch in the event of an open circuit condition. Other
safety features include inductor current limit, thermal shutdown, and an undervoltage lockout.
CIRCUIT COMPONENTS
Inductive DC/DC Boost Converter
In order to achieve the output voltages required to power high power white LEDs, the LM3551 and LM3552
utilize a highly efficient inductive DC/DC converter. The boost converter utilizes a current-mode controlled,
constant frequency (1.25MHz.), PWM architecture. This architecture creates a predictable noise spectrum that
allows for easy filtering and low noise. A very low on-resistance power NFET(RDSON = 0.165Ω) and high value
current limit (2.2A typ.) help efficiently provide a high power output (700mA@5V) over the entire lithium-ion
voltage range. The feedback voltage for both the LM3551 and LM3552 is tightly regulated to 1.265V.
SD/EN Pin
The LM3551 and LM3552 provide two different options in regards to turn-on control logic. The LM3551 utilizes a
shutdown pin (SD) that turns on the part when a voltage less than 0.3V is applied. An internal 1.2MΩpull-up to
VIN is provided to place the LM3551 into shutdown when no control signal is provided. The LM3552 utilizes a
enable pin (EN) that turns on the part when a voltage greater than 1.2V is applied. An internal 1.2MΩpull-down
to GND is provided to place the LM3552 into shutdown when no control signal is provided.
Low-side Load Disconnect FETs
The LM3551 and LM3552 have two low-side load disconnect NFETs (FET-T and FET-F) that provide the
physical mechanism of Torch Mode and Flash Mode. In Torch Mode, FET-T is enabled allowing current to flow
through it. FET-T has an on-resistance of 0.98Ωand is capable of handling currents up to 200mA. In Flash
Mode, both FET-T and FET-F are enabled. FET-F has an on-resistance of 0.36Ωand is capable of handling
currents up to 500mA. The total Flash current is equal to the sum total of the current flowing through FET-T and
FET-F. See the CURRENT SET EQUATIONS in the Application Informations section for more information
regarding setting LED current .
In shutdown mode, the LM3551/2 provide a true load disconnect helping to keep the total shutdown current to a
minimum.
Over-Voltage Protection (OVP)
The over voltage protection (OVP) is engaged when a failure mode occurs (FB pin grounded, Flash LED
becomes open or disconnected, etc.). In the event of a failure, OVP prevents the output voltage from exceeding
12.4V (typ). When the OVP level is reached, the switch FET shuts off preventing the output voltage from climbing
higher. Once the FET has shut off, the output will droop at a rate determined by the value of the output capacitor
and current leakage through the OVP pin and any other leakage path. When the output voltage drops to
10.6V(typ), switching will resume. The LM3551 and LM3552 will go back into OVP if the failure is still present
resulting in a pulsed output condition.
NOTE
To disable OVP, ground to OVP pin. CAUTION: The LM3551 and LM3552 may be
damaged if an OVP condition occurs and OVP is disabled.
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CFTO
SD
T/F
ITO
INTERNAL EN
T/F
INTERNAL EN
VCFTO
VTRIP
EXTERNAL/INTERNAL
FTO
SD / EN
SD / EN
EN
LM3551, LM3552
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Under-Voltage Protection (UVP)
Both the LM3551 and LM3552 have under-voltage protection circuitry (UVP). This protects the NMOS power
device during startup and shutdown by preventing operation at voltages less than the minimum input voltage.
The UVP protection is enabled at 2.48V(typ.) and will not disable until the input voltage rises above 2.58V(typ.) .
Torch/Flash Pin (T/F)
The TORCH/FLASH pin (T/F) controls whether the LM3551/2 is in continuous torch mode, or in flash mode. A
logic '0' places the part into torch mode and a logic '1' places the part into flash mode. There are no pull-ups or
pull-downs internally connected to T/F. When placed into torch mode, FET-T is enabled allowing the current set
by RTto flow. In torch mode, FET-F is not enabled. Flash mode enables both FET-T and FET-F allowing the sum
total of the current set by the two external resistors, RTand RF, to flow.
Flash Timeout Protection (FTP)
When SD is low(LM3551) or EN is high(LM3552), and T/F is high, a current is output to an external capacitor,
CFTO. This causes the voltage on the capacitor to rise. If the voltage reaches Vtrip (1.16V(typ)), the timeout circuit
forces the INTERNAL_EN signal to go low, which in turn shuts-off the low-side torch and flash FETs in addition
to disabling the main power SW FET. At such time, the LED will be turned off. The part will remain disabled until
SD is pulled high (LM3551) or EN is pulled low (LM3552) and/or T/F is pulled low. At that point, the part will
return to normal operating mode. The diagram below shows a first pulse which exceeds the timeout period and
internal_EN being driven low. The second FLASH pulse is shorter than the timeout period and therefore the
voltage on CFTO never reaches Vtrip. For information on component selection, please see the FLASH TIMEOUT
EQUATIONS below.
TFTO = CFTO × (ΔVFTO ÷ IFTO) (1)
ΔVFTO = 1.16V and IFTO= 1.4µA TFTO = Desired Timeout Duration (2)
CFTO(µF) = TFTO(sec.) × 1.21(µA/V) (3)
To disable the timeout function, ground the FTO pin.
Figure 15. Flash Timeout Protection Diagram
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Soft-Start
The LM3551 and LM3552 have a soft-start pin that can be used to limit the inductor inrush current on start-up.
The external SS pin is used to tailor the soft-start for a specific application but is not required for all applications
and can be left open when not needed. When used, a current source charges the external soft-start capacitor,
CSS, forcing the internal reference to ramp-up at a user determined rate.
Table 1. Typical Start-Up TimesVIN = 3.6V, TA= +25°C
CSS (µF) Load (mA) Start-Up Time (msec.)
Sharp LED @ 75mA Torch 3
Sharp LED @ 250mA Flash 8
0.1 Lumiled LED @ 200mA Torch 1.6
Lumiled LED @ 700mA Flash 6
Sharp LED @ 75mA Torch 12
Sharp LED @ 250mA Flash 35
0.47 Lumiled LED @ 200mA Torch 6
Lumiled LED @ 700mA Flash 35
Sharp LED @ 75mA Torch 25
Sharp LED @ 250mA Flash 75
1.0 Lumiled LED @ 200mA Torch 30
Lumiled LED @ 700mA Flash 70
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APPLICATION INFORMATION
LM3551 AND LM3552 FUNCTIONALITY TRUTH TABLE
SD (LM3551)
or T/F LM3551 Result LM3552 Result
EN (LM3552)
1 0 Shut-down Torch Mode
1 1 Shut-down Flash Mode
0 0 Torch Mode Shut-down
0 1 Flash Mode Shut-down
The LM3551 has a 1.2MΩpull-up to VIN on SD and the LM3552 has a 1.2MΩpull-down to GND on EN.
CURRENT SET EQUATIONS
The LM3551/2 utilize inline resistors to set the Torch and Flash LED currents. The Torch-Mode current
(continuous) and the Flash-Mode current (pulsed) are programmed by placing the appropriately selected
resistors between the feedback pin (FB) and FET-T (torch FET) and FET-F (flash FET) pins. Torch-mode is set
by utilizes the current through one resistor while Flash-mode is set by utilizes the currents though both current
set resistors. The following equations are used to set the LED currents.
RTORCH = (1.265V / ITORCH)-RDSON-T (4)
RFLASH = (1.265V / (IFLASH- ITORCH))-RDSON-F (5)
RDSON-T= 0.98Ωand RDSON-F = 0.36Ω(6)
NOTE
Flash LEDs from different manufacturers can have very different continuous and pulse
current ratings. See the manufacturers datasheets to ensure that the proper current levels
are used to avoid damaging the flash LED.
INDUCTOR SELECTION
Special care must be taken when selecting an inductor for use in LM3551/2 applications. The inductor should
have a current saturation rating that is larger than the worst case peak inductor current of the application to
ensure proper operation. Using an inductor with a lower saturation current rating than is required can cause a
dramatic drop in the inductance and can derate the maximum output current levels severely. It is worth noting
that the output voltage ripple is also affected by the total ripple current in the inductor. The following equations
can help give a good approximation as to what the peak inductor current will be for a given application at room
temperature (TA= +25°C).
IL(average) = [ILED × VOUT-MAX] ÷ [VIN-MIN × Eff.] (7)
ΔIL= [VIN × D] ÷ [L × FSW] (8)
IL(peak) = IL(ave) + [ΔIL÷ 2] (9)
VOUT-MAX Maximum Output Voltage. Maximum output voltage over temperature with OVP used is 11V (12.4V
typically).
VIN-MIN Minimum Input Voltage. Recommended minimum input voltage is 3.0V. The LM3551/2 will work down to
2.7V however, use at lower input voltages will required an inductor with a higher saturation current rating.
Eff. Converter Efficiency (approx. 85% over input voltage range).
DDuty Cycle = 1 - [VIN / VOUT]
LInductance. Recommended inductance value is 4.7µH.
FSW Switching Frequency = 1.25MHz
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Hz
1
fZ1 =2SRESRCOUT
Hz
1
fP1 =2S>RESR + (VOUT/ILED)]COUT
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DIODE SELECTION
The output diode for a boost regulator must be chosen correctly depending on the output voltage and output
current. The output diode must have a reverse voltage rating equal to or greater than the output voltage used.
The average current rating must be greater than the maximum load current expected, and the peak current rating
must be greater than the peak inductor current. Using Schottky diodes with lower forward voltage drop will
decrease power dissipation and increase efficiency.
CAPACITOR SELECTION
Input Capacitor
An input capacitor is required to reduce the input ripple and noise for proper operation of the regulator. The size
used is dependant on the application and board layout. If the regulator will be loaded uniformly, with very little
load changes, and at lower current outputs, the input capacitor size can often be reduced. The size can also be
reduced if the input of the regulator is very close to the source output. The size will generally need to be larger
for applications where the regulator is supplying nearly the maximum rated output or if large load steps are
expected. A minimum value of 10µF should be used under normal operating condtions while a 10-22µF capacitor
may be required for higher power and dynamic loads. Larger values and/or lower ESR may be needed if the
application requires very low ripple on the input source voltage.
Output Capacitor
A minimum output capacitor value of 4.7µF (Sharp LED) and 10µF (Lumiled) is recommended and may be
increased to a larger value. The ESR of the output capacitor is important because it determines the peak to peak
output voltage ripple according to the approximate equation:
ΔVOUT ≊2 × ΔIL× RESR (in Volts) (10)
After choosing the output capacitor you can determine a pole-zero pair introduced into the control loop by the
following equations:
(11)
The zero created by the ESR of the output capacitor is generally at a very high frequency if the ESR is small. If
low ESR capacitors are used it can be neglected. The output capacitor pole information is useful in selecting the
proper compensation components and is discussed in the Compensation Components section of the datasheet.
Capacitor Properties
Surface-mount multi-layer ceramic capacitors are recommended for both the input and output capacitors. These
capacitors are small, inexpensive and have very low equivalent series resistance (ESR <20mΩtyp.). Tantalum
capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are not recommended for use with the
LM3551/2 due to their high ESR, as compared to ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with
the LM3551/2. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over
temperature (X7R: ±15% over -55°C to 125°C; X5R: ±15% over -55°C to 85°C).
Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the
LM3551/2. Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -
20%) and vary significantly over temperature (Y5V: +22%, -82% over -30°C to +85°C range; Z5U: +22%, -56%
over +10°C to +85°C range). Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a
capacitance of only 0.1µF. Such deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum
capacitance requirements of the LM3551/2.
The minimum voltage rating acceptable for the input capacitor is 6.3V (10V recommended) and 16V for the
output capacitor. In applications that have DC operating points near the maximum voltage rating of the ceramic
capacitor, larger capacitor values may be required to compensate for capacitance loss due to capacitor voltage
coefficient. See the capacitor manufacturer's datasheet for DC bias performance.
12 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated
Product Folder Links: LM3551 LM3552
(in Hz)
RHPzero = VOUT(D')2
2S,LOADL
m1
#
VINRDSON
L(in V/s)
n = 1+ 2mc
m1 (no unit)
Leff = L
(D')2
Zc(in rad/s)
2fs
nD'
#
ADC(DB) = 20log10 {[(ZcLeff)// RL]//RL}(in dB)
RFB1 + RFB2
RFB2
()gmROD'
RDSON
LM3551, LM3552
www.ti.com
SNVS371D –AUGUST 2005–REVISED MAY 2013
COMPENSATION
DC Gain and Open-Loop Gain
Since the control stage of the converter forms a complete feedback loop with the power components, it forms a
closed-loop system that must be stabilized to avoid positive feedback and instability. A value for open-loop DC
gain will be required, from which you can calculate, or place, poles and zeros to determine the crossover
frequency and the phase margin. A high phase margin (greater than 45°) is desired for the best stability and
transient response. For the purpose of stabilizing the LM3551/2, choosing a crossover point well below where the
right half plane zero is located will ensure sufficient phase margin.
To ensure a bandwidth of ½ or less of the frequency of the RHP zero, calculate the open-loop DC gain, ADC.
After this value is known, you can calculate the crossover visually by placing a −20dB/decade slope at each pole,
and a +20dB/decade slope for each zero. The point at which the gain plot crosses unity gain, or 0dB, is the
crossover frequency. If the crossover frequency is less than ½ the RHP zero, the phase margin should be high
enough for stability. The equation for ADC is given below with additional equations required for the calculation:
(12)
(13)
(14)
(15)
mc ≊0.072 × fs (in V/s) (16)
(17)
where RLis the minimum load resistance, fs is the switching frequency, VIN is the minimum input voltage, gmis
the error amplifier transconductance and RDSON-S is the power switch on-resistance. The value for gmand RDSON-S
are found in the Electrical Characteristics table.
Right Half Plane Zero
A current mode control boost regulator has an inherent right half plane zero (RHP zero). This zero has the effect
of a zero in the gain plot, causing an imposed +20dB/decade on the rolloff, but has the effect of a pole in the
phase, subtracting another 90° in the phase plot. This can cause undesirable effects if the control loop is
influenced by this zero. To ensure the RHP zero does not cause instability issues, the control loop should be
designed to have a bandwidth of less than ½ the frequency of the RHP zero. This zero occurs at a frequency of:
(18)
where ILOAD is the maximum load current.
Copyright © 2005–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LM3551 LM3552
Hz
1
fZC =2SRCCC
Hz
1
2S(RC + RO)CC
fPC =
LM3551, LM3552
SNVS371D –AUGUST 2005–REVISED MAY 2013
www.ti.com
Compensation Components
The LM3551 and LM3552 provide a compensation pin (VC) to customize the voltage loop feedback. It is
recommended that a series combination of RCand CCbe used for the compensation network, as shown in the
typical application circuit. For any given application, there exists a unique combination of RCand CCthat will
optimize the performance of the LM3551/2 circuit in terms of its transient response. The series combination of RC
and CCintroduces a pole-zero pair. The frequency of the pole created is determined by the equation:
(19)
where ROis the output impedance of the error amplifier, approximately 900kΩ. Since RCis generally much less
than RO, it has little effect on the above equation and can be neglected until a value is chosen to set the zero fZC.
fZC is created to cancel the pole created by the output capacitor, fP1. The output capacitor pole will shift with
different load currents as shown by the equation, so setting the zero is not exact. Determine the range of fP1 over
the expected loads and then set the zero fZC to a point approximately in the middle. The frequency of this zero is
determined by:
(20)
Now RCcan be chosen with the selected value for CC. Check to make sure that the pole fPC is still in the 10Hz to
500Hz range, and change each value slightly if needed to ensure both component values are in the
recommended range. For both typical applications circuits shown on the front page, the Recommended value
for CCis 4.7nF and RC= 10kΩfor Lumiled applications. 10nF and 27kΩare recommended for Sharp
applications.
RECOMMENDED MINIMUM COMPONENT SPECIFICATIONS
Component Value Ratings
L1 4.7µH 2.0A 30% ISAT Rating
4.7µF (Sharp)
CIN 10V X5R or X7R
10µF (Lumiled)
4.7µF (Sharp)
COUT 16V X5R or X7R
10uF (Lumiled)
CFTO User Determined 6.3V X5R or X7R
4.7nF
(Lumiled)
CC6.3V X5R or X7R
10nF
(Sharp)
10kΩ
(Lumiled)
RC27kΩ
(Sharp)
RFLASH User Determined Application Specific
RTORCH User Determined Application Specific
CSS User Determined 6.3V X5R or X7R
Torch and Flash Resistor ratings are dependent upon the current through each resistor. The minimum ratings will
vary depending upon the current selected on an applicaiton by application basis. Power Rating Minimum =
(Desired Current)2× Resistor Value.See the CURRENT SET EQUATIONS section to determine torch and flash
currents.
14 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated
Product Folder Links: LM3551 LM3552
LM3551/2
VIN
VC
GND
SW FB
FET-T
FET-F
-+
CIN COUT
RTRF
D1
RC
CC
VBAT
SS
OVP
CFTO
FTO
CSS
SD/EN T/F
Sharp
GM5BW05340A
Flash LED
4.7 PF
16V
10 PF
10V
4.7 PH
RC = 27 k:
CC= 10 nF
LM3551/2
SD/
EN T/F
VIN
VC
GND
SW FB
FET-T
FET-F
CIN COUT
RTRF
D1
RC
CC
VBAT
SS
OVP
CFTO
FTO
LUMILED
LXCL-PWF1
Flash LED
10 Fµ
16V
10 Fµ
10V
CSS
4.7 µH
RC= 10 kΩ
CC= 4.7 nF
+
–
LM3551, LM3552
www.ti.com
SNVS371D –AUGUST 2005–REVISED MAY 2013
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM3551/2 when the junction temperature exceeds +140°C (typ.).
This feature protects the device from being damaged by high die temperatures that might otherwise result from
excessive power dissipation. The device will recover and operate normally when the junction temperature falls
below +120°C (typ.). It is important that the board layout provide good thermal conduction to keep the junction
temperature within the specified operating ratings.
PCB LAYOUT CONSIDERATIONS
The WSON is a leadframe based Chip Scale Package (CSP) with very good thermal properties. This package
has an exposed DAP (die attach pad) at the center of the package measuring 2.6mm x 3.0mm. The main
advantage of this exposed DAP is to offer lower thermal resistance when it is soldered to the thermal land pad
on the PCB. For PCB layout, Texas Instruments highly recommends a 1:1 ratio between the package and the
PCB thermal land. To further enhance thermal conductivity, the PCB thermal land may include vias to a ground
plane. For more detailed instructions on mounting WSON packages, please refer to Application Note AN-1187
(SNOA401)
Application Examples
TYPICAL CONFIGURATIONS
Figure 16. LUMILED LXCL-PWF1
RT= 5.6Ω, RF= 2.2Ω
ITORCH = 200mA, IFLASH = 700mA
Figure 17. SHARP GM5BW05340A
RT= 17Ω, RF= 6.5Ω
ITORCH = 75mA, IFLASH = 250mA
Copyright © 2005–2013, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LM3551 LM3552
LM3551/2
SD/EN T/F
VIN
VC
GND
SW
FB
FET-T
FET-F
-+
CIN COUT
R2RF
D1
L1
RC
CC
VBAT
SS
OVP
CFTO
FTO
R1
VOUT = 5V
LM3551, LM3552
SNVS371D –AUGUST 2005–REVISED MAY 2013
www.ti.com
DUAL-MODE CONFIGURATION(1)
Figure 18. R1= 29.5kΩ,R2= 10kΩRF= 3Ω
IFLASH = 500mA
Using the Dual-Mode configuration with either the LM3551/2, a 5V, high current rail (approx. 700mA total) can be
created while still allowing for a high flash with a true load disconnect. R1 and R2 setup the +5V following the
equation:VOUT = 1.265 × (1 + R1/R2) . When the LM3551/2 is on, and the T/F pin is low (logic '0'), the part will
provide a regulated output voltage that can be used to provide a voltage rail within a system. By setting the T/F
pin high, the LM3551/2 will allow the current to flow through the flash LED while still maintaining the fixed output
voltage rail. The flash current is set by RFusing the equation RF= [(VOUT - VLED)/ILED] - 0.36Ω, and should not
exceed 500mA. The total usable output current is dependent upon the output voltage selected. If the dual-mode
configuration is used, the FTO pin should be grounded to prevent the voltage rail from being shutdown at an
unwanted time.
(1) Please refer to the RECOMMENDED MINIMUM COMPONENT SPECIFICATIONS section of the datasheet for more information.
16 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated
Product Folder Links: LM3551 LM3552
LM3551, LM3552
www.ti.com
SNVS371D –AUGUST 2005–REVISED MAY 2013
REVISION HISTORY
Changes from Revision C (May 2013) to Revision D Page
• Changed layout of National Data Sheet to TI format .......................................................................................................... 16
Copyright © 2005–2013, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LM3551 LM3552
PACKAGE OPTION ADDENDUM
www.ti.com 16-Oct-2015
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
LM3551SD/NOPB LIFEBUY WSON NHL 14 1000 Green (RoHS
& no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 85 L3551SD
LM3552SD/NOPB LIFEBUY WSON NHL 14 1000 Green (RoHS
& no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 85 L3552SD
(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) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(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.
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.
PACKAGE OPTION ADDENDUM
www.ti.com 16-Oct-2015
Addendum-Page 2
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
LM3551SD/NOPB WSON NHL 14 1000 178.0 12.4 4.3 4.3 1.3 8.0 12.0 Q1
LM3552SD/NOPB WSON NHL 14 1000 178.0 12.4 4.3 4.3 1.3 8.0 12.0 Q1
PACKAGE MATERIALS INFORMATION
www.ti.com 2-Sep-2015
Pack Materials-Page 1
*All dimensions are nominal
Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)
LM3551SD/NOPB WSON NHL 14 1000 210.0 185.0 35.0
LM3552SD/NOPB WSON NHL 14 1000 210.0 185.0 35.0
PACKAGE MATERIALS INFORMATION
www.ti.com 2-Sep-2015
Pack Materials-Page 2
MECHANICAL DATA
NHL0014B
www.ti.com
SDA14B (Rev A)
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