Figure 10. Embedded high voltage start-up circuitry

As soon as the voltage on the HV pin is higher than VHV_START (29 V typ.), the HV-start-up circuitry turns on

and starts to source an IHV_ON current (7 mA typ.). This current, minus the device consumption ISTART_UP before

startup (400 µA typ.), charges the external bypass capacitor connected between VCC pin and ground increasing

its voltage almost linearly up to the device turn-on threshold VCC_ON (14 V typ.). To protect the HV-start-up

circuitry from excessive power consumption, e.g. for a short-circuit on the VCC pin, the IHV_ON current sourced is

1 mA typical till the VCC pin voltage is lower than VCC_SO (1 V typ.).

Once the VCC pin voltage reaches VCC_ON the device starts operating and to guarantee a reliable startup of the

system, the HV current generator IHV_ON is kept on till the output voltage reaches 70% typ. of the programmed

value (FB pin higher than VFB_HVSTUP threshold). To protect the HV-start-up circuitry from excessive power

consumption, e.g. in case of an overcurrent request, the HV current generator is in any case automatically turned

off after the TTOUT timeout (100 ms typ.)

After the start-up phase, the system application should guarantee VCC > VCC_OFF. In fact, if the VCC pin voltage

drops 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 operations performing the soft-start.

5.4

Input line discharge (X-cap discharge function)

In order to guarantee safety regulations such as IEC 61010-1/IEC 62368-1 without using the traditional discharge

resistor in parallel to the X-capacitor, and thus saving also the associated power losses enabling ultra-low

consumption in standby conditions, the device embeds an internal logic circuit that detects when the unit has

been detached from the power line; if this event occurs then the high voltage start-up generator is turned on to

discharge the X capacitors of the EMI filter to a safe level.

In particular, after a detection time TDECT_XCAP (64 ms typ.) from the AC mains disconnection, the X-cap

discharge operation is triggered and the internal HV current generator is turned on: a discharge current IHV_ON

(5 mA minimum) is drawn from the HV pin ensuring the X-cap discharge until the voltage on the HV pin falls

below a safe level (45 V maximum, VHV_MIN parameter in Section 4 Electrical characteristics, Table 5), within the

regulation maximum discharging time.

5.5

Soft-start

To limit the in-rush current of the converter at the startup, the device implements a soft startup increasing the peak

of the inductor current from zero up to the required value programmed by the control loop to regulate the output

voltage.

In particular, the device changes the reference threshold of the first overcurrent comparator (OCP1) from zero up

to VCS_OCP1 (-0.49 V typ.) in TSS_OCP1 time (130 ms typ.), as shown in Figure 11 .

L4985

Input line discharge (X-cap discharge function)

DS13769 - Rev 3

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