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

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

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

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

formation on component selection, please see the FLASH

TIMEOUT EQUATIONS below.

20151204

Flash Timeout Protection Diagram

T

∆V

∆V

T

C

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

internal reference to ramp-up at a user determined rate.

Typical Start-Up Times

V

C

Load (mA)

Start-Up Time

(msec.)

0.1

3

Flash

8

Torch

1.6

Flash

6

0.47

12

Flash

35

Torch

6

Flash

35

1.0

25

Flash

75

Torch

30

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 1.2M

Ω pull-up to V

LM3552 has a 1.2M

Ω 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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