LM3554 datasheet(26/43 Pages) TI1

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STROBE

TX1/TORCH

Timeout

Duration

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

(TX1/TORCH pin is a hardware torch input)

FLASH

TORCH

ILED

STROBE

TX1/TORCH

Timeout

Duration

Configuration Register 2 bit [2] = 1 (AET)

(STROBE goes high before TX1)

ITORCH

FLASH

ILED

LM3554

SNVS549A – JUNE 2009 – REVISED APRIL 2011

www.ti.com

Figure 14. Alternative External Torch Mode (STROBE Goes High Before TX1/TORCH, Same as Default

with SEM = 0)

Figure 15. TX1/TORCH Configured as a Hardware Torch input

FLAGS REGISTER AND FAULT INDICATORS

The Flags Register (0xD0) contains the Interrupt and Fault indicators. Five fault flags are available in the

LM3554. These include a Thermal Shutdown, an LED Failure Flag (LEDF) , a Timeout indicator Flag (TO), a

LED Thermal Flag (NTC), and a VIN Monitor Flag. Additionally, two interrupt flag bits TX interrupt and TX2

interrupt indicate a change of state of the TX1/TORCH pin (TX1 mode) and ENVM/TX2 pin (TX2 mode). Reading

back a "1" indicates the TX lines have changed state since the last read of the Flags Register. A read of the

Flags Register resets these bits.

Thermal Shutdown

When the LM3554’s die temperature reaches +150°C the boost converter shuts down, and the NFET and PFET

turn off. Additionally, all three current sources (LED1, LED2, and LEDI) turn off. When the thermal shutdown

threshold is tripped a (1) gets written to bit [1] of the Flag Register (Thermal Shutdown bit). The LM3554 will start

up again when the die temperature falls to below +135°C.

During heavy load conditions when the internal power dissipation in the device causes thermal shutdown, the

part will turn off and start up again after the die temperature cools. This will result in a pulsed on/off operation.

The OVT bit however will only get written once. To reset the OVT bit pull HWEN low, power down the LM3554,

or read the Flags Register.

LED Fault

The LED Fault flag (bit 2 of the Flags Register) reads back a (1) if the part is active in Flash or Torch mode and

either LED1 or LED2 experience an open or short condition. An LED open condition is signaled if the OVP

threshold is crossed at OUT while the device is in Flash or Torch mode. An LED short condition is signaled if the

voltage at LED1 or LED2 goes below 500mV while the device is in Torch or Flash mode.

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