LM3551/LM3552
1A White LED Driver with Flash Timeout Protection
General Description
The LM3551 and LM3552 are fixed frequency, current mode
step-up DC/DC converters with two integrated NFETs that
can be used for precision LED brightness control. The de-
vices are capable of driving loads up to 1A from a single-cell
Li-Ion battery.
The LM3551 and LM3552 can drive one or more high current
flash LEDs either in a high power Flash mode or a lower
power Torch mode using the TORCH/FLASH pin. A program-
mable Timeout function on the FTO pin forces the internal
NFETs to turn off after a certain user defined time. An
external SD pin (LM3551) or EN pin (LM3552) is available to
put the device into low power shutdown mode. During shut-
down, the feedback resistors and the load are disconnected
from the input to avoid leakage current paths to ground.
User programmable soft-start circuitry has been integrated
to eliminate large inrush currents at start-up. Over-voltage
protection circuitry and a 1.25MHz switching frequency allow
for the use of small, low-cost output capacitors with lower
voltage ratings.
The LM3551 and LM3552 are available in a low profile 14 pin
LLP package.
Features
nUp to 1A total drive current
nFlash timeout protection
nIndependent Torch/Flash/Shutdown modes
nLED disconnect in shutdown
nProgrammable soft-start limits inrush current
nOver-voltage protection
nWide voltage range 2.7 to 5.5V
n1.25MHz constant switching frequency
nSmall, low profile package, non-pullback LLP14 (4mm x
4mm)
Applications
nWhite LED Camera Flash
nWhite LED Torch (Flashlight)
nDSC (Digital Still Camera) Flash
nCellular Camera Phone Flash
nPDA Camera Flash
nCamcorder Torch (Flashlight) lamp
Typical Application Circuits
20151205
February 2006
LM3551/LM3552 1A White LED Driver with Flash Timeout Protection
© 2006 National Semiconductor Corporation DS201512 www.national.com
Connection Diagram
14 Pin Dual LLP Package
20151202
NS Package Number SDA14B
Pin Descriptions
Pin Name Function
9V
IN
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
3V
C
Compensation network connection. Connected to the output of the voltage error amplifier.
5SD(LM3551)
EN(LM3552)
Shutdown pin logic input. High = Shutdown, Low = Enabled
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
6 AGND
Analog Ground. Connect the ground of the compensation components, C
FTO
and soft start
cap to AGND. AGND must be connected to the GND pin through a low impedance
connection.
Ordering Information
Order Number SD / EN Package Marking Supplied As
LM3551SD SD L3551SD 250 units, Tape-and-Reel
LM3551SDX SD L3551SD 3000 units, Tape-and-Reel
LM3552SD EN L3552SD 250 units, Tape-and-Reel
LM3552SDX EN L3552SD 3000 units, Tape-and-Reel
LM3551/LM3552
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Absolute Maximum Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
IN
pin: Voltage to GND 7.5V
SW pin: Voltage to GND 21V
FB pin: Voltage to GND 7V
V
C
pin: 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 (Note 3) Internally Limited
Junction Temperature (T
J-MAX )
150˚C
Storage Temperature Range -65˚C to +150
Maximum Lead Temperature
(Soldering) (Note 4)
ESD Rating(Note 5)
Human Body Model 2.0kV
Operating Ratings(Notes 1, 2)
Input Voltage Range 2.7V to 5.5V
SW Voltage Max. (Note 6) 20V
Junction Temperature (T
J
)
Range
-40˚C to +110˚C
Ambient Temperature (T
A
)
Range (Note 7)
-40˚C to +85˚C
Thermal Properties
Junction-to-Ambient Thermal
Resistance (θ
JA
), SDA14B
Package (Note 8) 37.3˚C/W
ESD Caution Notice National Semiconductor recommends that all integrated circuits be handled with appropri-
ate ESD precautions. Failure to observe proper ESD handling techniques can result in damage to the device.
Electrical Characteristics(Notes 2, 9)
Limits in standard typeface are for T
J
= +25˚ C. Limits in boldface type apply over the full operating junction temperature range
(-40˚C ≤T
J
≤+110˚C). Unless otherwise noted, specifications apply to the LM3551 and LM3552 Typical Application Circuit (pg.
1) with: V
IN
= 3.6V, V(SD) = 0V for LM3551 and V(EN) = V
IN
for LM3552, I
LOAD
= 0A (Note 10)
Symbol Parameter Conditions Min Typ Max Units
I
Q
Quiescent Current FB = V
IN
(Not Switching) 1.47 2.0 mA
I
SD
Shutdown Current
V(SD) = V
IN
LM3551 2.55 5.0
µA
V(EN) = 0V
LM3552 0.1 2.3
I
CL(Note 11)
Switch Current Limit V
IN
= 3.0V(Note 12) 2.1 A
V
FB
Feedback Voltage 1.2285 1.265 1.2915 V
I
FB(Note 13)
Feedback Pin Bias Current 50 nA
g
m
Error Amp
Transconductance ∆I = 5µA 135 µmho
A
V
Error Amp Voltage Gain 135 V/V
D
MAX
Maximum Duty Cycle 92.5 %
f
sw
Switching Frequency 0.9 1.25 1.6 MHz
I
SDPIN
Shutdown Pin Current
(LM3551) V
SD
= 0V 3.0 6µA
I
ENPIN
Enable Pin Current
(LM3552) V
EN
= 3.6V 3.0 6µA
I
T/FPIN
T/F Pin Current V
T/F
=0V 26 nA
V
T/F
=V
IN
22
I
L-SW
SW Pin Leakage Current V
L-SW
= 20V 0.07 8µA
R
DSON-SW
SW Pin R
DSON
I
SW
= 0.5A 0.165 Ω
I
L-T
FET-T Leakage Current 0.1 µA
R
DSON-T
FET-T R
DSON
0.98 Ω
I
L-F
FET-F Leakage Current 0.1 µA
R
DSON-F
FET-F R
DSON
0.36 Ω
Th
SD/EN
Shutdown/Enable Pin
Threshold
Output High 1.2 V
Output Low 0.3
LM3551/LM3552
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Electrical Characteristics(Notes 2, 9) (Continued)
Limits in standard typeface are for T
J
= +25˚ C. Limits in boldface type apply over the full operating junction temperature range
(-40˚C ≤T
J
≤+110˚C). Unless otherwise noted, specifications apply to the LM3551 and LM3552 Typical Application Circuit (pg.
1) with: V
IN
= 3.6V, V(SD) = 0V for LM3551 and V(EN) = V
IN
for LM3552, I
LOAD
= 0A (Note 10)
Symbol Parameter Conditions Min Typ Max Units
Th
T/F
T/F Pin Threshold Output High 1.2 V
Output Low 0.3
UVP Under Voltage Protection
Thresholds
On Threshold 2.25 2.48 2.70 V
Off Threshold 2.43 2.58 2.77
OVP Over Voltage Protection
Thresholds
On Threshold 11.3 12.4 14 V
Off Threshold 9.2 10.6 12
V
FTO
Flash Timeout trip-point 0.99 1.16 1.32 V
I
FTO
Flash Timeout Current 1.12 1.4 1.68 µA
V
SS
Soft-Start Voltage 1.18 1.25 1.32 V
I
SS
Soft-Start Current 10 11.5 13 µA
Note 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 guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the
Electrical Characteristics tables.
Note 2: All voltages are with respect to the potential at the GND pin.
Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=140oC (typ.) and disengages at
TJ=120oC (typ.).
Note 4: For detailed soldering specifications and information, please refer to National Semiconductor Application Note: AN-1187 for Recommended Soldering
Profiles.
Note 5: The human body model is a 100pF capacitor discharged through a 1.5kΩresistor into each pin. (MIL-STD-883 3015.7)
Note 6: Maximum recommended SW pin voltage when the OVP pin is grounded.
Note 7: 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 =110
oC), 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 =T
J-MAX-OP –(θJA xP
D-MAX).
Note 8: 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.
Note 9: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical (Typ) numbers are not guaranteed, but do represent the most likely norm.
Unless otherwise specified, conditions for Typ specifications are: VIN = 3.6V and TA=25
oC.
Note 10: CIN and COUT,: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics
Note 11: Duty cycle affects current limit due to ramp generator.
Note 12: Current limit at 0% duty cycle. See TYPICAL PERFORMANCE section for Switch Current Limit vs. VIN
Note 13: Bias current flows into FB pin.
LM3551/LM3552
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Typical Performance Characteristics Unless otherwise specified: T
A
= +25˚C; V
IN
= 3.6V; L =
4.7µH, (R
C
= 10kΩ,C
C
= 4.7nF, C
IN
=C
OUT
= 10µF for Lumiled LED), (R
C
= 27kΩ,C
C
= 10nF, C
IN
= 10µF, C
OUT
= 4.7µF
for Sharp LED), C
FTO
= 1µF, C
SS
= 0.1µF.
Current Limit vs. Input Voltage
V
OUT
=5V
Current Limit vs. Input Voltage
V
OUT
= 10V
20151220
Note 14: ICL measure when VOUT =95%xV
OUT (nominal)
20151223
Note 15: ICL measure when VOUT =95%xV
OUT (nominal)
Converter Efficiency vs. Input Voltage
Lumiled Flash LED
Converter Efficiency vs. Input Voltage
Sharp Flash LED
20151211 20151212
LM3551/LM3552
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Typical Performance Characteristics Unless otherwise specified: T
A
= +25˚C; V
IN
= 3.6V; L = 4.7µH,
(RC= 10kΩ,C
C
= 4.7nF, C
IN
=C
OUT
= 10µF for Lumiled LED), (R
C
= 27kΩ,C
C
= 10nF, C
IN
= 10µF, C
OUT
= 4.7µF for Sharp
LED), C
FTO
= 1µF, C
SS
= 0.1µF. (Continued)
Maximum I
OUT
vs. Input Voltage
V
OUT
=5V
Maximum I
OUT
vs. Input Voltage
V
OUT
= 10V
20151222
Note 16: IOUT measured at 95%x VOUT (nominal)
20151224
Note 17: IOUT measured at 95%x VOUT (nominal)
LED Torch Current vs. Input Voltage
Lumiled Flash LED
LED Flash Current vs. Input Voltage
Lumiled Flash LED
20151218 20151219
LM3551/LM3552
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Typical Performance Characteristics Unless otherwise specified: T
A
= +25˚C; V
IN
= 3.6V; L = 4.7µH,
(RC= 10kΩ,C
C
= 4.7nF, C
IN
=C
OUT
= 10µF for Lumiled LED), (R
C
= 27kΩ,C
C
= 10nF, C
IN
= 10µF, C
OUT
= 4.7µF for Sharp
LED), C
FTO
= 1µF, C
SS
= 0.1µF. (Continued)
OVP Trip Voltage vs. Input Voltage Switching Frequency vs. Input Voltage
20151217 20151221
Start-Up Waveform
Sharp LED
Start-Up Waveform
Lumiled LED
20151232
Ch1=V
SD
,Ch3=I
LED
,Ch4=I
IN
20151235
Ch1=V
SD
,Ch3=I
LED
,Ch4=I
IN
LM3551/LM3552
www.national.com7
Typical Performance Characteristics Unless otherwise specified: T
A
= +25˚C; V
IN
= 3.6V; L = 4.7µH,
(RC= 10kΩ,C
C
= 4.7nF, C
IN
=C
OUT
= 10µF for Lumiled LED), (R
C
= 27kΩ,C
C
= 10nF, C
IN
= 10µF, C
OUT
= 4.7µF for Sharp
LED), C
FTO
= 1µF, C
SS
= 0.1µF. (Continued)
Typical Switching Waveform
20151233
Sharp LED in Flash Mode
Ch1=V
SW
,Ch3=I
LED
,Ch4=I
L
LM3551/LM3552
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Block Diagram
20151203
Circuit Description
OVERVIEW
The LM3551/2 are high power white LED flash drivers ca-
pable 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 lock-
out.
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 con-
verter 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(R
DSON
=
0.165Ω) and high value current limit (2.2A typ.) help effi-
ciently 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 shut-
down pin (SD) that turns on the part when a voltage less than
0.3V is applied. An internal 1.2MΩpull-up to V
IN
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 mecha-
nism 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 ca-
pable 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 EQUA-
TIONS in the Application Informations section for more infor-
mation regarding setting LED current .
In shutdown mode, the LM3551/2 provide a true load dis-
connect 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
LM3551/LM3552
www.national.com9
Circuit Description (Continued)
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.
Under-Voltage Protection (UVP)
Both the LM3551 and LM3552 have under-voltage protec-
tion 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 R
T
to 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, R
T
and R
F
, 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, C
FTO
. 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 C
FTO
never reaches Vtrip. For in-
formation on component selection, please see the FLASH
TIMEOUT EQUATIONS below.
20151204
Flash Timeout Protection Diagram
T
FTO
=C
FTO
x(∆V
FTO
÷I
FTO
)
∆V
FTO
= 1.16V and I
FTO
= 1.4µA
T
FTO
= Desired Timeout Duration
C
FTO
(µF) = T
FTO
(sec.) x 1.21(µA/V)
To disable the timeout function, ground the FTO pin.
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, C
SS
, forcing the
internal reference to ramp-up at a user determined rate.
Typical Start-Up Times
V
IN
= 3.6V, T
A
= +25˚C
C
SS
(µF) Load (mA) Start-Up Time
(msec.)
0.1
Sharp LED @75mA Torch 3
Sharp LED @250mA
Flash 8
Lumiled LED @200mA
Torch 1.6
Lumiled LED @700mA
Flash 6
0.47
Sharp LED @75mA Torch 12
Sharp LED @250mA
Flash 35
Lumiled LED @200mA
Torch 6
Lumiled LED @700mA
Flash 35
1.0
Sharp LED @75mA Torch 25
Sharp LED @250mA
Flash 75
Lumiled LED @200mA
Torch 30
Lumiled LED @700mA
Flash 70
Application Information
LM3551 AND LM3552 FUNCTIONALITY TRUTH TABLE
SD (LM3551)
or
EN (LM3552)
T/F LM3551 Result LM3552 Result
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 V
IN
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
LM3551/LM3552
www.national.com 10
Application Information (Continued)
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.
R
TORCH
= (1.265V / I
TORCH
)-R
DSON-T
R
FLASH
= (1.265V / (I
FLASH
-I
TORCH)
)-R
DSON-F
R
DSON-T
= 0.98Ωand R
DSON-F
= 0.36Ω
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 induc-
tor. The following equations can help give a good approxi-
mation as to what the peak inductor current will be for a
given application at room temperature (T
A
= +25˚C).
I
L
(average) = [I
LED
xV
OUT-MAX
]÷[V
IN-MIN
x Eff.]
∆I
L
=[V
IN
xD]÷[LxF
SW
]
I
L
(peak) = I
L
(ave) + [∆I
L
÷2]
V
OUT-MAX
Maximum Output Voltage. Maximum output volt-
age over temperature with OVP used is 11V
(12.4V typically).
V
IN-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 volt-
ages will required an inductor with a higher satu-
ration current rating.
Eff. Converter Efficiency (approx. 85% over input volt-
age range).
DDuty Cycle=1-[V
IN
/V
OUT
]
LInductance. Recommended inductance value
is 4.7µH.
F
SW
Switching Frequency = 1.25MHz
DIODE SELECTION
The output diode for a boost regulator must be chosen
correctly depending on the output voltage and output cur-
rent. The output diode must have a reverse voltage rating
equal to or greater than the output voltage used. The aver-
age 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 di-
odes 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 regu-
lator 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:
∆V
OUT
)2x∆I
L
xR
ESR
(in Volts)
After choosing the output capacitor you can determine a
pole-zero pair introduced into the control loop by the follow-
ing equations:
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 COM-
PENSATION COMPONENTS section of the datasheet.
Capacitor Properties
Surface-mount multi-layer ceramic capacitors are recom-
mended for both the input and output capacitors. These
capacitors are small, inexpensive and have very low equiva-
lent series resistance (ESR <20mΩtyp.). Tantalum capaci-
tors, OS-CON capacitors, and aluminum electrolytic capaci-
tors 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 toler-
ance (as good as ±10%) and hold their value over tempera-
ture (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 capaci-
tor 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 capaci-
LM3551/LM3552
www.national.com11
Application Information (Continued)
tance loss due to capacitor voltage coefficient. See the
capacitor manufacturer’s datasheet for DC bias perfor-
mance.
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 feed-
back 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
1
⁄
2
or less of the frequency of the
RHP zero, calculate the open-loop DC gain, A
DC
. 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
1
⁄
2
the RHP zero, the phase
margin should be high enough for stability. The equation for
A
DC
is given below with additional equations required for the
calculation:
mc )0.072 x fs (in V/s)
where R
L
is the minimum load resistance, fs is the switching
frequency, V
IN
is the minimum input voltage, g
m
is the error
amplifier transconductance and R
DSON-S
is the power switch
on-resistance. The value for g
m
and R
DSON-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
1
⁄
2
the frequency of the RHP zero. This zero occurs at a fre-
quency of:
where I
LOAD
is the maximum load current.
Compensation Components
The LM3551 and LM3552 provide a compensation pin (V
C
)
to customize the voltage loop feedback. It is recommended
that a series combination of R
C
and C
C
be used for the
compensation network, as shown in the typical application
circuit. For any given application, there exists a unique com-
bination of R
C
and C
C
that will optimize the performance of
the LM3551/2 circuit in terms of its transient response. The
series combination of R
C
and C
C
introduces a pole-zero pair.
The frequency of the pole created is determined by the
equation:
where R
O
is the output impedance of the error amplifier,
approximately 900kΩ. Since R
C
is generally much less than
R
O
, it has little effect on the above equation and can be
neglected until a value is chosen to set the zero f
ZC
.f
ZC
is
created to cancel the pole created by the output capacitor,
f
P1
. 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 f
P1
over the expected loads
and then set the zero f
ZC
to a point approximately in the
middle. The frequency of this zero is determined by:
Now R
C
can be chosen with the selected value for C
C
.
Check to make sure that the pole f
PC
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 C
C
is 4.7nF and
R
C
= 10kΩfor Lumiled applications. 10nF and 27kΩare
recommended for Sharp applications.
RECOMMENDED MINIMUM COMPONENT
SPECIFICATIONS
Component Value Ratings
L1 4.7µH 2.0A 30% I
SAT
Rating
C
IN
4.7µF (Sharp) 10V X5R or
X7R
10µF (Lumiled)
C
OUT
4.7µF (Sharp) 16V X5R or
X7R
10uF (Lumiled)
C
FTO
User
Determined
6.3V X5R or
X7R
LM3551/LM3552
www.national.com 12
Application Information (Continued)
Component Value Ratings
C
C
4.7nF
(Lumiled) 6.3V X5R or
X7R
10nF
(Sharp)
R
C
10kΩ
(Lumiled)
27kΩ
(Sharp)
R
FLASH
User
Determined
Application
Specific
R
TORCH
User
Determined
Application
Specific
C
SS
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 Cur-
rent)
2
x Resistor Value. See the CURRENT SET EQUA-
TIONS section to determine torch and flash currents.
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 nor-
mally 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 LLP 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, National 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 mount-
ing LLP packages, please refer to National Semiconductor
Application Note AN-1187.
Application Examples
TYPICAL CONFIGURATIONS
20151215
LUMILED LXCL-PWF1
R
T
= 5.6Ω,R
F
= 2.2Ω
I
TORCH
= 200mA, I
FLASH
= 700mA
20151216
SHARP GM5BW05340A
R
T
=17Ω,R
F
= 6.5Ω
I
TORCH
= 75mA, I
FLASH
= 250mA
Note 18: Please refer to the RECOMMENDED MINIMUM COMPONENT
section of the datasheet for more information.
DUAL-MODE CONFIGURATION
20151209
R
1
= 29.5kΩ,R
2
= 10kΩR
F
=3Ω
I
FLASH
= 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 discon-
nect. R1 and R2 setup the +5V following the equation: V
OUT
= 1.265 x (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
LM3551/LM3552
www.national.com13
Application Examples (Continued)
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 R
F
using the equation R
F
= [(V
OUT
-V
LED
)/I
LED
]-
0.36Ω, and should not exceed 500mA. The total usable
output current is dependent upon the output voltage se-
lected. 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.
LM3551/LM3552
www.national.com 14
Physical Dimensions inches (millimeters) unless otherwise noted
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
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Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain
no ‘‘Banned Substances’’ as defined in CSP-9-111S2.
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www.national.com
LM3551/LM3552 1A White LED Driver with Flash Timeout Protection
