LM3556

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SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015

Feature Description (continued)

7.3.3.6 Undervoltage Lockout (UVLO)

The LM3556 has an internal comparator that monitors the voltage at IN and forces the LM3556 into shutdown if

the input voltage drops to 2.8 V. If the UVLO monitor threshold is tripped, the UVLO flag bit is set in the Flags

read command initiated for the Flags Register (0x0B). Upon a read, the Flags Register is cleared, and normal

operation can resume. This feature can be disabled by writing a '0' to the UVLO EN bit in the Input Voltage Flash

Monitor (IVFM) Mode Register (0x01). The mode bits in the Enable Register (0x0A) are cleared upon a UVLO

event.

When the LM3556 device’s die temperature reaches 150°C, the boost converter shuts down, and the NFET and

PFET turn off, as does the current source (LED). When the thermal shutdown threshold is tripped, a '1' gets

LM3556 can only restart after the Flags Register (0x0B) is read, clearing the fault flag. Upon restart, if the die

temperature is still above 150°C, the device resets the fault flag and re-enters standby. The mode bits in the

The LED fault flag in the Flags Register (0x0B) reads back a '1' if the part is active in Flash Mode or Torch Mode,

condition if the OVP threshold is crossed at the OUT pin while the device is in Flash or Torch Mode. An LED

short condition is determined if the voltage at LED goes below 500 mV (typical) while the device is in either

Torch or Flash Mode. There is a delay of 256-

μs deglitch time before the LED flag is valid, and 2.048 ms before

LED voltage and the output voltage are sampled. The LED flag can only be reset to '0' by removing power to the

LM3556, or by reading back the Flags Register (0x0B). The mode bits in the Enable Register (0x0A) are cleared

7.4 Device Functional Modes

7.4.1 Start-Up (Enabling The Device)

Turnon of the LM3556 Torch and Flash Modes can be done through the Enable Register (0x0A). On start-up,

(typical) to the output capacitor. During this time the current source (LED) is off. When the voltage across the

output capacitor reaches 2.2 V (typical) the current source turns on. At turnon the current source steps through

each FLASH or TORCH level until the target LED current is reached. This gives the device a controlled turnon

7.4.2 Pass Mode

The LM3556 starts up in Pass Mode and stays there until Boost Mode is needed to maintain regulation. If the

current must be limited to 2 A.

7.4.3 Flash Mode

In Flash Mode, the LED current source (LED) provides 16 target current levels from 93.75 mA to 1500 mA. The

Flash currents are adjusted via the Current Control Register (0x09). Flash mode is activated by the Enable

Register (0x0A), or by pulling the STROBE pin HIGH. Once the Flash sequence is activated the current source

(LED) ramps up to the programmed Flash current by stepping through all current steps until the programmed

current is reached.

When the device is enabled in Flash Mode through the Enable Register, all mode bits in the Enable Register are

cleared after a flash time-out event.

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