Figure 11. Soft-start circuitry details

5.6

No load operation (burst-mode function)

To avoid uncontrolled rise of the output voltage at light or zero load, when VCOMP < VCOMP_S (1 V typ.) the device

stops the switching activity and reduces its power consumption. As soon as the VCOMP > VCOMP_R (1.05 V typical

and in tracking with the threshold VCOMP_S) the device restarts the switching activity.

If the burst-mode condition is triggered when the gate driver GD is “high”, the device completes the current

ON-time and the system stoppage takes place after GD falling edge.

During the burst-mode operation, the system application should guarantee VCC > VCC_OFF. In fact, if the VCC pin

voltage drops down to VCC_OFF, the device shutdown and the HV-startup is turned on to bring the VCC pin to a

voltage VCC_ON and restart the operation performing the soft-start.

5.7

Idle operation (external burst-mode function)

The FB pin can be used to implement an external burst-mode (EBM), forcing the pin lower than the internal

threshold VFB_FF/EBM (500 mV typ.): the switching activity is stopped and the IC power consumption is reduced

(Error Amplifier is also turned off and the COMP pin is forced to “high impedance”). The device restarts switching,

without implementing the soft-start, as soon as the FB voltage exceeds the VFB_FF/EBM threshold by 50 mV typ.

De-bounce time TFF/EBM_DB (1.8 µs typ.) is provided to avoid false triggering. Referring to Figure 12, it is worth

noting that as soon as the device enters the EBM state (the internal signal EBM_INT is high) a weak pull-up

current IFB_EBM (100 µA typ.) is sourced from the FB pin in order to speed up the FB voltage rising edge when the

external pull-down is released. In addition, once the FB voltage exceeds VFB_FF/EBM + 50 mV the IFB_EBM current

of 100 µA is increased to 1 mA till the FB pin voltage reaches the final target of 2.5 V (internal VREF). When using

the EBM function, the suggested value of the FB filter capacitor CFB is 3.3 nF.

L4985

No load operation (burst-mode function)

DS13769 - Rev 3

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