AP3984

Document number: DS37655 Rev. 3 - 2

7 of 15

www.diodes.com

December 2016

© Diodes Incorporated

AP3984

Constant Voltage Operation

The AP3984 captures the auxiliary winding feedback voltage at FB pin and operates in constant-voltage (CV) mode to regulate the output voltage.

Assuming the secondary winding is master, the auxiliary winding is slave during the D1 on-time. The auxiliary voltage is given by:





D

S

AUX

AUX

V

V

N

N

V







O

See equation 5

0V

Figure 3. Auxiliary Voltage Waveform

voltage is always detected at a constant secondary current, the difference between the output voltage and the secondary voltage will be a fixed

regulate the output voltage.

Constant Current Operation

The AP3984 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

4

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







4

1