LM3561

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SNOSB44C – MARCH 2011 – REVISED MAY 2013

CURRENT LIMIT

compatible interface via bit [5] of the Flash Duration Register. When the current limit is reached the LM3561

stops switching for the remainder 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

operates in Pass Mode. In situations where there could potentially be large load currents at OUT, and the

LM3561 is operating in Pass mode, the load current must be limited to 1.5A. In Boost mode or Pass mode if

the current to typically 200mA. This prevents damage to the LM3561 and excessive current draw from the battery

during output short circuit conditions.

THERMAL SHUTDOWN

The LM3561 features a thermal shutdown threshold of typically +150°C. When the die temperature reaches

+150°C, the active current source (LED) will shutdown, and the TSD flag in the Flags register is written high. The

device cannot be started up again until the Flags register is read back. Once the Flags register is read back the

current source can be re-enabled into Torch, or Flash Mode. The thermal shutdown (TSD) circuitry has an

internal 250µs de-glitch timer which helps prevent unwanted noise from falsely triggering a TSD event. However,

when the LM3561 is in boost mode at higher flash currents, the de-glitch timer can get reset by the high currents

in the LM3561's GND. As a result the thermal shutdown's internal de-glitch timer can get reset before the TSD

event can get latched in. This causes a TSD event to not get triggered until the LM3561's flash pulse reaches the

end of the flash duration, when the noisy currents have dropped to a lower level. However, once the noise is

lower and a TSD event is triggered, the next flash pulse is not allowed until the flags register is read back. In

pass mode the boost switcher is off and the lower noise environment allows the devices TSD circuitry to

shutdown immediately when the die temperature reaches +150°C.

FLASH MODE

In Flash mode the LED current source (LED) provides 16 different current levels from typically 36mA to 600mA.

The Flash currents are set by writing to bits [3:0] of the Flash Brightness Resister. Flash mode is activated by

either writing a (1, 1) to bits [1:0] of the Enable Register, 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 Torch and Flash levels (32µs/step) until the programmed current is reached.

FLASH TERMINATION

Bit [2] of the Enable Register determines how the Flash pulse terminates. With this bit = '1' the Flash current

pulse will only terminate by reaching the end of the Flash timeout period (see Figure 29). With STR = '0', Flash

mode can be terminated by pulling STROBE low, programming bits [1:0] of the Enable Register with (0,0), or by

allowing the Flash timeout period to elapse (see Figure 28). If STR = '0' and STROBE is toggled before the end

of the Flash timeout period, the timeout period resets on the rising edge of STROBE. See LM3561 Timing

Diagrams regarding the Flash pulse termination for the different STR bit settings.

compatible interface, the current source (LED) turns completely off. This happens even when Torch is enabled

(bits [1:0] of the Enable Register are automatically re-written with (0, 0).

FLASH TIMEOUT

The Flash timeout period sets the maximum duration of the flash current pulse. Bits [4:0] of the Flash Duration

Register programs the 32 different Flash timeout levels in steps of 32ms, giving a Flash timeout range of 32ms to

1024ms (see Table 10).

TORCH MODE

In Torch mode the current source (LED) provides 8 different current levels (see Table 8). The Torch currents are

adjusted by writing to bits [2:0] of the Torch Brightness Register. Torch mode is activated by setting Enable

Register bits [1:0] to (1, 0). Once the Torch mode is enabled the current sources will ramp up to the programmed

Torch current level by stepping through all of the Torch currents at (32µs/step) until the programmed Torch

current level is reached.

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