LM3551 Datasheet, PDF (10/15 Pages) National Semiconductor (TI) – 1A White LED Driver with Flash Timeout Protection

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.
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.)
Sharp LED @ 75mA Torch
3
Sharp LED @ 250mA
8
Flash
0.1
Lumiled LED @ 200mA
1.6
Torch
Lumiled LED @ 700mA
6
Flash
Sharp LED @ 75mA Torch
12
Sharp LED @ 250mA
35
Flash
0.47
Lumiled LED @ 200mA
6
Torch
Lumiled LED @ 700mA
35
Flash
Sharp LED @ 75mA Torch
25
Sharp LED @ 250mA
75
Flash
1.0
Lumiled LED @ 200mA
30
Torch
Lumiled LED @ 700mA
70
Flash
Application Information
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)
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LM3551 AND LM3552 FUNCTIONALITY TRUTH TABLE
SD (LM3551)
or
EN (LM3552)
1
1
0
0
T/F LM3551 Result LM3552 Result
0
Shut-down
Torch Mode
1
Shut-down
Flash Mode
0
Torch Mode
Shut-down
1
Flash Mode
Shut-down
The LM3551 has a 1.2MΩ pull-up to V_IN on SD and the
LM3552 has a 1.2MΩ pull-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
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