AP1684A

Document number: DS37095 Rev. 1 - 2

9 of 13

www.diodes.com

August 2014

© Diodes Incorporated

AP1684A

A Product Line of

Diodes Incorporated

Application Information (Cont.)

Valley on Control Method

The valley on function can provide low turn-on switching losses for buck converter. The voltage across the drain and source of the power mosfet is

reflected by the auxiliary winding of the buck transformer. The voltage is sensed by the FB pin.

0.1V

1µs

Valley

FB

Figure 2. Valley on Control

According to Figure 2, when the falling edge of 0.1V is sensed by the FB pin, the AP1684A will see the tOFF time is over and delay 1µs to start a

new operating cycle. In this way we can realize valley on function.

Fault Protection

Over Voltage Protection and Output Open Protection

VCC

FB

R5

R6

AP1684A

Figure 3. OVP Circuit

The output voltage is sensed by the auxiliary winding voltage of the buck transformer, the VCC pin and FB pin provide over voltage protection

function. When the output is open or large transient happens, the output voltage will exceed the rated value. When the voltage of VCC cap exceeds

Vcc_ovp or VFB_CV, the over voltage is triggered and the IC will discharge VCC. When the VCC is below the UVLO threshold voltage, the IC will start

a new work cycle and the VCC cap is charged again by start resistance. If the over voltage condition still exists, the system will work in hiccup

mode.

Output Short Protection

When the output is shorted, the output voltage will be clamped at 0. At this condition, VCC will drop down without auxiliary winding for power supply.

And the VCC will drop to UVLO threshold voltage, the IC will shut down and restart a new operating cycle, and the VCC is charged by startup

resistance. When the VCC is higher than Vcc_start voltage, the IC will output a bunch of pulse to control power mosfet on and off, which will

consume the energy stored in the VCC cap, because of no VCC supply from the auxiliary winding, the VCC will drop down to VCC UVLO threshold

voltage again. If output short condition still exists, the system will operate in hiccup mode.

Over Temperature Protection

The AP1684A has two kinds of over temperature protection processes. First, the system is operating normally, the ambient temperature is

changed to +170°C suddenly, the IC will trigger over temperature protection which leads to a latch work mode. Second, if the system starts, the

over temperature protection will be triggered when the ambient temperature is higher than +150°C. So the AP1684A can startup successfully

when the ambient temperature is less than +150°C.