ALTAIR04-900TR データシートの表示（PDF） - STMicroelectronics

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ALTAIR04-900TR

Off-line all-primary-sensing switching regulator

STMicroelectronics 

ALTAIR04-900

Application information

Figure 10. Timing diagram: normal power-up and power-down sequences

Vin

V Start

Vcc

t

VccON

Vccresta rt

t

DRAIN

5.3

Icharg e

5.5 mA

Po wer - on

Normal operation

CV mode

Normal operation

CC mode

tt

t

P owe r- o ff

Zero-current detection and triggering block

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

negative-going edge falling below 50 mV is applied to the ZCD/FB pin. The triggering block

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

This feature detects transformer demagnetization for QR operation, where the signal for

ZCD input is obtained by the transformer auxiliary winding, also used to supply the IC.

Figure 11. ZCD block, triggering block

R zcd ZCD/FB

Rf b

Aux

ZCD

CLAMP

110mV

60mV

BLAN KIN G

TI ME

STAR TER

Fr om CC /CV Block

TU RN -ON

LOGI C

LEB

S

Q

R

F rom OC P

To D riv er

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

following leakage inductance demagnetization, from triggering the ZCD circuit erroneously.

This blanking time is dependent on the voltage on COMP pin: it is TBLANK = 30 µs for

VCOMP = 0.9 V, and decreases almost linearly down to TBLANK = 6 µs for VCOMP = 1.3 V.

The voltage on the pin is both top and bottom-limited by a double clamp, as illustrated in the

internal diagram of ZCD block (see Figure 11). The upper clamp is typically 3.3 V, while the

lower clamp is -60 mV. The interface between the pin and the auxiliary winding is a resistor

DocID18211 Rev 3

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