AL1771 datasheet(12/24 Pages) DIODES

12 / 24 page

AL1771/1772

Document number: DS41314 Rev. 1 - 2

12 of 24

www.diodes.com

August 2018

AL1771/1772

Functional Description (Cont.)

Figure 6. Auxiliary Voltage Waveform

Figure 6 shows the voltage waveform of the auxiliary winding. To accurately sample the divided auxiliary winding voltage, the FB pin delays a

tSAMPLE time before sampling. And tDISCHARGE is the demagnetization time for the transformer.

Load Transition Operation

To obtain good load transition performance, the COMP regulation mechanism is optimized. When the load changes from heavy to light causing

the FB voltage reaching VFB_OVP (typically 1.35V), the IC will pull down the COMP voltage and enter the max-off-time mode, accelerating the

discharge of the output voltage. When the load changes from light to heavy leading to the FB voltage touching 1.0V, the IC will charge up the

COMP voltage and raise the MOSFET on time, quickening the increase of the output voltage.

1.35V

VFB

VCS

VCOMP

Heavy load transition to light load

max-off-time mode

0 V

VFB

VCS

VCOMP

Light load transition to heavy load

1.0 V

0 V

Figure 7. Load Transition Waveform

Protections

Output-Open Protection (AVCC_OVP; VFB_OVP)

The output voltage is reflected by the voltage on tr

ansformer’s auxiliary winding. Both FB pin and AVCC pins have OVP function.

When there is a rapid line and load transient, the output voltage may exceed the regulated value. If VFB exceeds VFB_OVP, the OVP will be

triggered, and then PFC controller increases the OFF time to reduce output voltage.

If AVCC exceeds VAVCC_OVP the OVP will be triggered, the switch will be turned off and AVCC will be discharged. Once AVCC is below

VAVCC_OPR_MIN, the PFC controller will shut down and power on again by BUS voltage through start up resistor.

Output Short Protection (OSP)

When the output is shorted, the output voltage is clamped to zero. The output voltage of the auxiliary winding, which is proportional to the

secondary winding, will drop down too. Once AVcc is below VAVCC_OPR_MIN, the IC will shut down and power on again by the BUS voltage through

the startup resistor.

AL1771 Datasheet(PDF) 12 Page - Diodes Incorporated