AP3988

Document number: DS36722 Rev. 4 - 2

8 of 12

www.diodes.com

June 2014

© Diodes Incorporated

AP3988

A Product Line of

Diodes Incorporated

Constant Current Operation

The AP3988 is designed to work in constant current (CC) mode. Figure 4 shows the secondary current waveforms.

0A

See equation 7

Figure 4. Secondary Current Waveform

or charging the built-in capacitance connected. This fixed proportion is

3

4



offs

ons

T

T

offs

ons

ons

PKS

OUT

T

T

T

I

I









2

1

At the instant of D1 turn-on, the primary current transfers to the secondary at an amplitude of:

PK

S

P

PKS

I

N

N

I





Thus the output constant current is given by:

PK

S

P

OUT

I

N

N

I







7

2

Leading Edge Blanking (LEB)

When the power switch is turned on, a turn-on spike on the output pulse rising edge will occur on the sense-resistor. To avoid false termination of

the switching pulse, a typical 750ns leading edge blanking is built in. During this blanking period, the current sense comparator is disabled and the

gate driver cannot be switched off.

The built-in LEB in AP3988 has shorter delay time from current sense terminal to output pulse than those IC solutions adopting external RC filter

as LEB.

Built-in Cable Compensation

The AP3988 has built-in fixed voltage of 0.35V typical to compensate the drop of output cable when the load is changed from zero to full load. A

typical 10nF external capacitor connected to the CPC pin is used to smooth voltage signal for cable compensation.

Over Temperature Protection

The AP3988 has internal thermal sensing circuit to shut down the PFM driver output when the die temperature reaches +160°C typical. When the

die temperature drops about +40°C, the IC will recover automatically to normal operation.