LM3556

SNVS796C – AUGUST 2011 – REVISED APRIL 2013

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Flash Time-Out

The Flash Time-Out period sets the amount of time that the Flash Current is being sourced from the current

source (LED). The LM3556 has 8 time-out levels ranging 100 ms to 800 ms in 100 ms steps. The Flash Time-

Out period is controlled in the Flash Features Register (0x08). Flash Time-Out only applies to the Flash Mode

operation. The mode bits are cleared upon a Flash Time-out.

Over-Voltage Protection (OVP)

Threshold”, the LM3556 will begin switching again. The mode bits in the Enable Register (0x0A) are not cleared

upon an OVP.

Current Limit

The LM3556 features selectable inductor current limits that are programmable through the Flash Features

terminate the charging phase of the switching cycle.

Since the current limit is sensed in the NMOS switch, there is no mechanism to limit the current when the device

PFET operates as a current source limiting the current to 200 mA. This prevents damage to the LM3556 and

excessive current draw from the battery during output short-circuit conditions. The mode bits in the Enable

Register (0x0A) are not cleared upon a Current Limit event.

NTC Thermistor Input (TEMP)

The TEMP pin serves as a threshold detector for negative temperature coefficient (NTC) thermistors. It interrupts

the LED current when the voltage at TEMP goes below the programmed threshold. The NTC threshold voltage is

adjustable from 200 mV to 900 mV in 100 mV steps. The NTC current is adjustable from 25 µA to 100 µA in 25

µA steps. When an over-temperature event is detected, the LM3556 can be set to force the LED current from

Flash Mode into Torch Mode or into shutdown. These settings are adjusted via the NTC Settings Register (0x02),

and the NTC detection circuitry can be enabled or disabled via the Enable Register (0x0A). If enabled, the NTC

block will turn on and off during the start and stop of a Flash/Torch/Indicator event. The NTC mode of operation

is set by adjusting the NTC Mode bit in the Configuration Register (0x07). See the NTC Settings Register (0x02)

section for more details. The mode bits in the Enable Register (0x0A) are cleared upon an NTC event.

Under-Voltage Lockout (UVLO)

The LM3556 has an internal comparator that monitors the voltage at IN and will force the LM3556 into shutdown

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

Flags Register (0x0B). If the input voltage rises above 2.8V, the LM3556 will not be available for operation until

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.

Thermal Shutdown (TSD)

When the LM3556’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 written to

the corresponding bit of the Flags Register (0x0B) (Thermal Shutdown bit), and the LM3556 will go into standby.

The LM3556 will only be allowed to restart after the Flags Register (0x0B) is read, clearing the fault flag. Upon

restart, if the die temperature is still above +150°C, the LM3556 will reset the Fault flag and re-enter standby.

The mode bits in the Enable Register (0x0A) are cleared upon a TSD.

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