AS3642 datasheet(12/27 Pages) AMSCO

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AS3642

Datasheet - D e ta i l e d D e s c r i p t i o n

Protection and Fault Detection Functions

The protection functions protect the AS3642 and the LED(s) against physical damage. In most cases a register bit is

set, which can be readout with the interface. The fault bits are cleared

3 by a readout of the fault register.

DCDC Overvoltage Protection

In case of no or a broken LED at the pin LED_OUT and an enabled DCDC converter, the voltage on VOUT rises until

it reaches VVOUTMAX (overvoltage condition) and the voltage across the current source does not reach regulation

(VOUT-VLED < VILED_COMP). If this condition is detected, the DCDC converter is stopped, the current sources are dis-

abled and the bit fault_ovp (see page 19)

4 is set.

DCDC Broken Coil Detection

If the coil LDCDC is broken, the AS3642 is not powered by the pin SW connected to VOUT by the internal switch. Due

to the protection diodes between SCL to VOUT and SDA to VOUT, the AS3642 can be powered through these diodes.

The AS3642 detects this error condition by comparing the voltage on SCL and VOUT. If the voltage on VOUT is lower

compared to the voltage on SCL, the AS3642 will ignore any I

2C write commands. Therefore the application can simply

detect this condition.

Note: Due to the broken coil detection, the high levels of SDA and SCL should be always below the supply voltage.

LED Short Circuit Protection

After the startup of the DCDC converter, the voltage on LED_OUT is continuously monitored and compared against

VLEDSHORT. If the voltage stays below VLEDSHORT, the DCDC is stopped (as a shorted LED is assumed), the current

sources are disabled and the bit fault_led_short (see page 19) is set.

Overtemperature Protection

The junction temperature of the AS3642 is continuously monitored. If the temperature exceeds TOVTEMP, the DCDC is

stopped, the current sources are disabled and the bit fault_overtemp (see page 19) is set. The driver cannot be re-

enabled unless the junction temperature drops below TOVTEMP-TOVTEMPHYST.

Flash Timer

The duration of the flash is defined by the register flash_timer (see page 18). After the timer expires, the DCDC is

stopped and the flash current source (on pin LED_OUT) is disabled.

Supply undervoltage Protection

If the voltage on the pin VOUT (=battery voltage) is or falls below VUVLO, the AS3642 is kept in shutdown state and in

all registers are set to their default state.

Note: During operation of the DCDC converter, the supply undervoltage protection will still monitor the DCDC output

voltage only. Therefore the supply undervoltage protection will only monitor the battery voltage if the DCDC

converter is switched off and the output capacitor is discharged down to the supply voltage.

Wakeup Circuit - Power off detection

In flash, assist light and indicator mode (register mode (page 18)=01, 10 or 11), if SCL and SDA are L for more than

tTIMEOUT, shutdown mode is automatically entered. This feature automatically detects a power-off of the controlling cir-

cuit driving SCL and SDA (VDD_I/F goes to 0V) - the internal circuit is shown in Figure 19:

3. Except overtemperature protection bit fault_overtemp: This bit can be cleared once the temperature drops

below TOVTEMP-TOVTEMPHYST.

4. In indicator mode (mode=01) the register fault_ovpi s never set (the overvoltage protection is nevertheless

operating)

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