LM3554 datasheet(23/43 Pages) TI1

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STROBE

TX1/TORCH

Timeout

Duration

Default State

(TX event during a STROBE event)

IFLASH

ITORCH

LED

STROBE

Timeout

Duration

I2C Torch

Command

Default State

Flash Brightness Register bit 7 (STR) = 0

Configuration Register 1 bit 7 (TX1/TORCH) = 0

Configuration Register 1 bit 6 (TX2 Polarity) = 1

Configuration Register bit 5 (ENVM/TX2) = 0

Configuration Register 2 bit 2 (AET) = 0

IFLASH

I TORCH

ILED

LM3554

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SNVS549A – JUNE 2009 – REVISED APRIL 2011

With Configuration Register 2 bit [2] set to (1) the operation of TX1/TORCH becomes dependent on its

occurrence relative to STROBE. In this mode if TX1/TORCH goes high first, then STROBE goes high, the LEDs

are forced into Torch mode with no timeout. In this mode if TX1/TORCH goes high after STROBE has gone high

then the TX1/TORCH pin operates as a normal TX interrupt, and the LEDs will turn off at the end of the timeout

duration. (See LM3554 TIMING DIAGRAMS, Figure 13, and Figure 14.)

INPUT VOLTAGE MONITOR

The LM3554 has an internal comparator that monitors the voltage at IN and can force the LED current into Torch

mode or into shutdown if VIN falls below the programmable VIN Monitor Threshold. Bit 0 in the VIN Monitor

of 3.1V, 3.2V, 3.3V, and 3.4V. Bit 3 in Configuration Register 2 (0xF0) selects whether an under-voltage event

forces Torch mode or forces the LEDs off. See Figure 24/Table 7 and Figure 26/Table 9 for additional

information.

There is a set 100mV hysteresis for the input voltage monitor. When the input voltage monitor is active, and VIN

falls below the programmed VIN Monitor Threshold, the LEDs will either turn off or their current will get reduced

to the programmed Torch current setting. To reset the LED current to its previous level, two things must occur.

First, VIN must go at least 100mV above the UVLO threshold and secondly, the Flags register must be read back.

LM3554 TIMING DIAGRAMS

Figure 6. Normal Torch to Flash Operation (Default, Power On or RESET state of LM3554)

Figure 7. TX1 Event During a Flash Event (Default State,TX1/TORCH is an Active High TX Input)

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