AL3644

Document number: DS41558 Rev. 1 - 2

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July 2019

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AL3644

3.5 Fault / Protections

3.5.1 Fault Operation

If the AL3644 enters a fault condition, the device sets the appropriate flag in the Flags1 and Flags2 Registers (0x0A and 0x0B), and place the

device into standby by clearing the Mode Bits ([1],[0]) in the Enable Register. The AL3644 remains in standby until an I

Flags2 Registers are completed. Upon clearing the flags/faults, the device can be restarted (Flash, Torch, IR, etc.). If the fault is still present, the

AL3644 re-enters the fault state and enters standby again.

3.5.2 Flash Time-Out

The Flash Time-Out period sets the amount of time that the Flash Current is being sourced from the current sources (LED1/2). The AL3644 has

16 timeout levels ranging from 10ms to 400ms or 40ms to 1.6s on AL3644TT (see Timing Configuration Register (0x08) for more detail).

3.5.3 Overvoltage Protection (OVP)

bits are cleared, and the OVP flag is set, when an OVP condition is present for three rising OVP edges. This prevents momentary OVP events

from forcing the device to shut down.

3.5.4 Current Limit

The AL3644 features two selectable inductor current limits that are programmable through the I

limit is reached, the AL3644 terminates the charging phase of the switching cycle. Switching resumes at the start of the next switching period. If

the over current condition persists, the device operates continuously in current limit.

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 (current

operates as a current source limiting the current to 200mA. This prevents damage to the AL3644 and excessive current draw from the battery

during output short-circuit conditions. The mode bits are not cleared upon a Current Limit event, but a flag is set.

3.5.5 NTC Thermistor Input (TORCH/TEMP)

The TORCH/TEMP pin, when set to TEMP mode, serves as a threshold detector and bias source for negative temperature coefficient (NTC)

thermistors. When the voltage at TEMP goes below the programmed threshold, the AL3644 is placed into standby mode. The NTC threshold

voltage is adjustable from 200mV to 900mV in 100

mV steps. The NTC bias current is set to 50μA. The NTC detection circuitry can be enabled or

disabled via the Enable Register. If enabled, the NTC block turns on and off during the start and stop of a Flash/Torch event.

Additionally, the NTC input looks for an open NTC connection and a shorted NTC connection. If the NTC input falls below 100mV, the NTC short

flag is set, and the device is disabled. If the NTC input rises above 2.3V, the NTC Open flag is set, and the device is disabled. These fault

detections can be individually disabled/enabled via the NTC Open Fault Enable bit and the NTC Short Fault Enable bit.

Figure 6. Temp Detection Diagram