AP3127/H

Document number: DS37920 Rev. 2 - 2

8 of 13

www.diodes.com

September 2015

© Diodes Incorporated

AP3127/H

Operation Description

The AP3127/H is specifically designed for off-line AC-DC power supply used in LCD monitor, notebook adapter and battery charger applications.

It offers a cost effective solution with a versatile protection function.

Start-up Current and UVLO

The start-up current of AP3127/H is optimized to realize ultra low current (1

direct benefit of low start-up current is the availability of using large start-up resistor, which minimizes the resistor power loss for high voltage AC

input.

An UVLO comparator is included in AP3127/H to detect the voltage on VCC pin. It ensures that AP3127/H can draw adequate energy from hold-

up capacitor during power-on. The turn-on threshold is 16V and the turn-off threshold is 7.6V.

Current Sense Comparator and PWM Latch

The AP3127/H operates as a current mode controller, the output switch conduction is initiated by every oscillator cycle and is terminated when

the peak inductor current reaches the threshold level established by the FB pin. The inductor current signal is converted to a voltage signal by

S

FB

PK

R

V

I

3

/

)

8

.

0

(





Moreover, FOCP with 1.8V threshold is only about 100ns delay, which can avoid some catastrophic damages such as secondary rectifier short

test. Few drive cycles can alleviate the destruction range and get better protection.

Leading-edge Blanking

A narrow spike on the leading edge of the current waveform can usually be observed when the power MOSFET is turned on. A 250ns leading-

edge blank is built-in to prevent the false-triggering caused by the turn-on spike. During this period, the current limit comparator is disabled and

the gate driver can not be switched off.

At the time of turning off the MOSFET, a negative undershoot (maybe larger than -0.3V) can occur on the SENSE pin. So it is strongly

recommended to add a small RC filter or at least connect a resistor “R” on this pin to protect the IC (Shown as Figure 1).

FB

SENSE

GATE

GND

SOVP/

UVP

6

1

3

4

AP3127/H

VCC

5

2

Large undershoot (more than

-0.3V) may damage the SENSE pin

R

C

Necessary

Figure 1

Built-in Slope Compensation

It is well known that a continuous current mode SMPS may become unstable when the duty cycle exceeds 50%. The built-in slope compensation

can improve the stability, so there is no need for design engineer to spend much time on that.

FB Pin and Short Circuit Protection

This pin is normally connected to the opto-coupler and always paralleled with a capacitor for loop compensation. When the voltage at this pin is

decreasing to low UVLO level, then the IC will try to restart until the failure removed. And when this voltage is less than 1.55V, the IC will stop the

drive pulse immediately. Therefore, this feature can be used for short circuit protection, which makes the system immune from damage. Normally,