AP3988 Просмотр технического описания (PDF) - Diodes Incorporated.

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AP3988

PRIMARY SIDE POWER SWITCHER FOR OFF-LINE SMPS

Diodes Incorporated. 

Operation Description (Cont.)

A Product Line of

Diodes Incorporated

AP3988/89/90

Constant Current Operation

The AP3988/89/90 is designed to work in constant current (CC) mode. Figure 4 shows the secondary current waveforms.

See equation 7

Is

0A

IOUT

Tons

Toffs

Figure 4. Secondary Current Waveform

In CC operation, the CC loop control function of AP3988/89/90 will keep a fixed proportion between D1 on-time Tons and D1 off-time Toffs by

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

Tons  4 ……… (6)

Toffs 3

The relation between the output constant-current and secondary peak current IPKS is given by:

IOUT



1

2



I PKS

 Tons

Tons  Toffs

………(7)

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

I PKS



NP

NS

 IPK

………(8)

Thus the output constant current is given by:

IOUT



2

7

NP

NS

 IPK

………(9)

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 500ns 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 AP3988/89/90 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 AP3988/89/90 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 AP3988/89/90 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.

AP3988/89/90

Document number: DS36722 Rev. 3 - 2

8 of 13

www.diodes.com

March 2014

© Diodes Incorporated

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