LM3556

SNVS796C – AUGUST 2011 – REVISED APRIL 2013

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The LM3556 has three logic inputs including a hardware Flash Enable (STROBE), a hardware Torch Enable

(TORCH) used for external torch mode control and custom LED indication waveforms, and a Flash Interrupt input

(TX) designed to interrupt the flash pulse during high battery current conditions. All three logic inputs have

internal 300 k

Ω (typ.) pulldown resistors to GND.

Additional features of the LM3556 include an internal comparator for LED thermal sensing via an external NTC

current levels, changing the Flash Timeout Duration, changing the switch current limit, and enabling the NTC

block. Additionally, there are flag and status bits that indicate flash current time-out, LED over-temperature

Startup (Enabling the Device)

Turn on of the LM3556 Torch and Flash modes can be done through the Enable Register (0x0A). On startup,

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

capacitor reaches 2.2V (typ.) the current source will turn on. At turn-on the current source will step through each

FLASH or TORCH level until the target LED current is reached. This gives the device a controlled turn-on and

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 2A.

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) will ramp up to the programmed Flash current by stepping through all current steps until the programmed

current is reached.

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

cleared after a flash time-out event.

Data can be written to the mode bits (bits[1:0]) in Enable Register (0x0A) only after the flash has ramped down to

enable the Flash Mode.

Enable and Configuration Register

Status of Mode Bits in the Enable Register after a

Mode change required

Setting (0x0A=Enable Register,

flash.

0x07=Configuration Register)

Using Edge Triggered STROBE to

0x0A=0x23; 0x07=0x78 (default

Mode bits are cleared after a single flash. To reflash,

Flash

setting)

0x23 will have to be written to 0x0A.

Using Level Triggered STROBE to

0x0A=0x23; 0x07=0xF8

Mode bits are cleared after a single flash. To reflash,

Flash

0x23 will have to be written to 0x0A.

Part is required to go from External

0x0A=0x33; 0x07=0x78 (default

Mode bits are cleared after a single flash. To reflash,

TORCH Mode to External STROBE

setting)

0x33 will have to be written to 0x0A.

mode using Edge Triggered STROBE

Part is required to go from External

0x0A=0x33; 0x07=0xF8

Mode bits are cleared only if the part has an internal flash

TORCH Mode to External STROBE

time-out event happening before the STROBE level goes

mode using Level Triggered STROBE

low. To reflash, 0x33 will have to be written to 0x0A. If

the STROBE level goes low before an internal flash time-

out event, then mode bits are not cleared.

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