ALTAIR05T-800

Application information

Doc ID 17957 Rev 1

13/28

Figure 10.

Timing diagram: normal power-up and power-down sequences

5.3

Zero current detection and triggering block

The zero current detection (ZCD) and triggering blocks switch on the power MOSFET if a

negative-going edge falling below 50 mV is applied to the ZCD/FB pin. To do so, the

triggering block must be previously armed by a positive-going edge exceeding 100 mV.

This feature is used to detect transformer demagnetization for QR operation, where the

signal for the ZCD input is obtained from the transformer’s auxiliary winding used also to

supply the IC.

Figure 11.

ZCD block, triggering block

The triggering block is blanked after MOSFET’s turn-off to prevent any negative-going edge

that follows leakage inductance demagnetization from triggering the ZCD circuit

erroneously.

Vcc

DRAIN

t

tt

t

Vin

5.5 mA

t

t

Power-on

Power-off

Normal operation

CV mode

CC mode

Normal operation

Vcc

DRAIN

t

tt

t

Vin

5.5 mA

t

t

Power-on

Power-off

Normal operation

CV mode

CC mode

Normal operation

60mV

ZCD

CLAMP

BLANKIN G

TIME

TU RN- ON

LO GIC

STARTER

S

R

Q

LEB

Aux

Rfb

Rzcd

To Dr iv er

Fr om CC/ CV Block

Fr om OC P

ZCD/FB

110mV