ALTAIR04-900 Datasheet, PDF (13/29 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

ALTAIR04-900
Application information
Figure 10. Timing diagram: normal power-up and power-down sequences
Vin
V Start
Vcc
t
VccON
Vccresta rt
DRAIN
t
5.3
I charg 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 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
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’s turn-off to prevent any negative-going edge
that follows leakage inductance demagnetization from triggering the ZCD circuit
erroneously.
This blanking time is dependent on the voltage on COMP pin: it is T_BLANK = 30 µs for V_COMP
= 0.9 V, and decreases almost linearly down to T_BLANK = 6 µs for V_COMP = 1.3 V
Doc ID 18211 Rev 2
13/29