Application information

HVLED001B

22/33

DS12748 Rev 2

7.2.2

Feedback input

The OPTO pin is intended to be connected directly to the collector of the optocoupler that

provides the galvanic insulation to the control loop, while the FB pin is the output of the error

amplifier for the Primary side control loop of the output voltage (PSR) (see Section 7.2.4). A

suitable pick-up capacitor can be connected to the OPTO pin while suitable compensation

network for PSR is placed between FB and ground.

The FB voltage is also used as input parameter for burst mode operation described in the

relevant paragraph.

These pins embed a protection to stop the switching activity in case of excessive power

delivery (OPP protection).

7.2.3

Zero current detection

The zero level detection is performed by a trigger logic that operates as follows:

a)

The logic is armed if ZCD voltage is higher than VZCD,arm after Tblank,min

starting from GD turn-off instant.

b)

The logic is triggered to turn on the MOSFET when a falling edge crosses ZCD,trig

threshold. A small additional delay between ZCD triggering and GD turn-on is

there to turn on the MOSFET in correspondence if the bottom of resonance valley.

The advanced ZCD logic is able to discriminate between normal operation, output short-

circuit or start-up condition.

An internal blanking time prevents any triggering signal to activate the MOSFET at the very

beginning of the OFF-time, where spurious resonances could be present. As a result, the

first falling edge occurring after the blanking time turns on the MOSFET.

To ensure a proper operation, the transformer has to be designed to guarantee that the

inductor's demagnetization time is longer than TBLANK (@ VTOFF > VTOFF,fix) when the

The TOFF pin is intended to apply an additional blanking time following the first ZCD

triggering event. If the pin is left unconnected, null blanking time is provided. The

TBLANK,var value depends on TOFF voltage as illustrated in Figure 21.

An internal starter provides the triggering signal whenever a valid arming signal is not

detected.

The ZCD pin embeds a negative clamp to limit the negative going current.