AP8012H

13/F, Building C, Wangzhuang Technology Innovation Center, Longshan Road,New District, Wuxi Tel: +86(510)85217718 http://www.chipown.com Rev.A 1801 6 / 10

Functional Description

1. Startup

This device includes a high voltage start up current source connected on the SW of the device. As soon as a voltage

is applied on the input of the converter, this start up current source is activated and to charge the VDD capacitor as

long as VDD is lower than VSTART. When reaching VSTART, the start up current source is cut off and VDD is sourced

by auxiliary side. As VDD falls below VSTOP, the HV-Start circuit won’t work immediately until VDD is lower than

VRST.

Fig 2. Startup circuit

2. Soft-start up

In the process of start up, the current of drain increases to maximum limitation step by step. As a result, it can

reduce the stress of secondary diode greatly and help to prevent the transformer turning into the saturation states.

Typically, the duration of soft-start is 10 ms.

3. Gate driver

The internal power MOSFET in AP8012H is driven by a dedicated gate driver for power switch control. Too weak

the gate driver strength results in higher conduction and switch loss of MOSFET while too strong gate drive results

in worse EMI.

A good tradeoff is achieved through the built-in totem pole gate design with proper output strength and dead time.

The good EMI system design and low idle loss is easier to achieve with this dedicated control scheme.

4. Oscillator

The switching frequency of AP8012H is internally fixed at 45 kHz. No external frequency setting components are

required for PCB design.

The frequency modulation is implemented in AP8012H. So that, it minimizes the conduction band EMI and

therefore eases the system design because the tone energy could be spread out.

5. Feed-back

A feedback pin controls the operation of the device. Unlike conventional PWM control circuits which use a voltage

input, the COMP pin is sensitive to current. Figure 3 presents the internal current mode structure. The Power

MOSFET delivers a sense current which is proportional to the main current. R2 receives this current and the current

coming from the COMP pin. The voltage across R2 (VR2) is then compared to a fixed reference voltage. The

MOSFET is switched off when VR2 equals the reference voltage.