LM3555

SNVS594E – JANUARY 2009 – REVISED NOVEMBER 2011

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Flash Timeout (FTP)

mode, the FTP fault will be triggered whenever a flash is initiated through the Control Register (OEN and

OM1/OM0 bits) or through an edge-sensitive strobe event. A FTP fault could occur in Simple Control Mode if the

controller tied to EN1 and EN2 pins cannot toggle the pins low at the desired pulse rate. This same condition

could occur with a level-sensitive Strobe event controlled by a camera module. This fault is reported to the TO

fault bit in the Fault Register (bit4 in address 0x05). A FTP fault is the only reported "fault" that does not need to

be cleared before any additional LED event can occur.

Indicator Fault (IF)

An IF fault is triggered when the voltage on the IND pin is greater than 2.571V or less than 0.842V. This fault

indicates that their is either an open or a short present on the IND pin. The short circuit condition is cleared when

the indicator pin is allowed to operate between 0.842V and 2.571V. A shorted or open indicator LED could cause

this fault to occur. This fault is reported to the IF fault bit in the Fault Register (bit2 in address 0x05).

Broken Inductor Fault (IP)

An IP fault is triggered when the LM3555 detects the inductors inductance has dropped below an acceptable

value. This fault indicates that the inductor has been damaged. A inductor that has had its ferrite material

damaged could cause this fault to occur. This fault is reported to the IP fault bit in the Fault Register (bit1 in

address 0x05).

INDUCTOR CURRENT LIMIT

To prevent damage to the LM3555's inductor and to limit the power drawn by the LM3555 during a flash event,

an Inductor Current Limit circuit is present. The LM3555 monitors the current through the inductor during the

charge phase of the boost cycle. In the event that the inductor current reaches the current limit, the NFET of the

converter will terminate the charge phase for that cycle. The process will repeat itself until the flash event has

ended or until the input voltage increases to the point where the peak current is no longer reached. Hitting the

peak inductor current limit will not disable the part. It will however limit the output power delivery to the LEDs.

limit can be set to 1.25A, 1.5A, 1.75A and 2A depending on the values of the IL1 and IL0 bits in the Control

Register (Address 0x04). The peak inductor current limit value can be used to help size the inductor to the

appropriate saturation current level. For more information on inductor sizing, please refer to the INDUCTOR

SELECTION section of this datasheet.

UNDER-VOLTAGE LOCK-OUT (UVLO)

The LM3555 has an Under-Voltage Lock-Out (UVLO) feature that disables the parts operation in the event that

the input voltage falls below 2.4V (typ.). In Simple Control Mode, the input voltage must increase to at least

2.47V (typ.) and the EN1 and EN2 pins must be toggled low ('0') before normal operation can resume.

of a UVLO occurrence. The input voltage must rise to at least 2.47V before the LM3555 will become fully

functional again.

A UVLO event does not disturb the state of the other registers of the LM3555

POWER-ON RESET (POR)

the proper startup values once the input voltage reaches a voltage greater than 1.8V (typ.). An input voltage

lower than 1.8V will not only place the part into UVLO, but will also clear all of the LM3555 registers.

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