LM3554TMXSLASHNOPB datasheet(16/47 Pages) TI1

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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

SNVS549C – JUNE 2009 – REVISED FEBRUARY 2016

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Feature Description (continued)

device shuts down the current sources when VLEDI/NTC falls below VTRIP. When the LM3554 is forced from flash

into torch (by VLEDI/NTC falling below VTRIP), normal LED operation (during the same flash pulse) can only be re-

started by reading from the Flags Register (0xD0) and ensuring the voltage at VLEDI/NTC is above VTRIP. When

VLEDI/NTC falls below VTRIP, and the Flags register is cleared, the LM3554 goes through a 250-µs deglitch

time before the flash current falls to either torch mode or goes into shutdown.

7.3.11 Alternative External Torch (AET Mode)

Configuration Register 2 bit [2] programs the LM3554 for AET mode. With this bit set to 0 (default) TX1/TORCH

is a transmit interrupt that forces torch mode only during a flash event. For example, if TX1/TORCH goes high

during a flash event then the LEDs is forced into torch mode only for the duration of the timeout counter. At the

end of the timeout counter the LEDs turn off.

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 turn off at the end of the timeout

duration. (See LM3554 Timing Diagrams, Figure 35, and Figure 36.)

7.3.12 Input Voltage Monitor

The LM3554 has an internal comparator that monitors the voltage at IN, which 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

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

undervoltage event forces torch mode or forces the LEDs off. See /Table 7 and /Table 9 for additional

information.

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

falls below the programmed VIN monitor threshold, the LEDs either turn off or their current is 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 100 mV above the UVLO threshold and secondly, the Flags Register must be read back.

7.3.13 LM3554 Timing Diagrams

Figure 28. Normal Torch-to-Flash Operation (Default, Power On or LM3554 Reset State)

LM3554TMX/NOPB Datasheet(PDF) 16 Page - Texas Instruments

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