DocID027333 Rev 1

17/27

HVLED001

Application information

27

7.1.4

Low consumption mode

This state is intended to stop the switching activity reducing the power consumption to

a minimum level. The soft-start procedure is set to be performed when abnormal or

disabling condition is removed.

During this state the VCC is kept between VCC,su and VCC,on by the high voltage start-up

unit (HVSU) delivering Icharge to the output capacitor.

Note:

Important: HVSU charges VCC so any other external voltage (including auxiliary winding)

must be decoupled using a 1N4148 diode.

7.1.5

Soft-start

The soft-start phase is entered after the start-up and every time the IC exits from low

consumption mode. This phase lasts until the voltage at the CTRL pin reaches the “end of

soft-start” level (Veoss).

The current sense maximum limit is derived from this voltage, therefore the charging time of

a capacitor placed between the CTRL pin and GND defines the soft-start time.

During this phase some protections (optocoupler failure protection (OFP), brownout (BO)

and analog disable (AN_DIS) are ignored.

7.2

Control loop

The control loop is based on the current mode quasi-resonant flyback control scheme and is

therefore performed turning off the MOSFET when the peak of its source current reaches

the threshold set by the control loop, and turning the MOSFET on in correspondence of the

resonant valley following the primary side demagnetization input.

A detail of the block involved into this scheme is shown in Figure 8.

Figure 8. Control loop blocks