AP3968/69/70

Document number: DS36759 Rev. 2 - 2

8 of 15

www.diodes.com

December 2013

© Diodes Incorporated

AP3968/69/70

A Product Line of

Diodes Incorporated

Constant Voltage Operation

The AP3968/69/70 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

Vo

N

N

V







…………(5)

See equation 5

0V

Portion of Tons

Tons

Figure 3. Auxiliary Voltage Waveform

The voltage detection point is portion of Tons after D1 is turned on. The CV loop control function of AP3968/69/70 then generates a D1 off-time to

regulate the output voltage.

Constant Current Operation

The AP3968/69/70 is designed to work in constant current (CC) mode. Figure 4 shows the secondary current waveforms.

Is

0A

See equation 8

Tons

Toffs

Iout

Figure 4. Secondary Current Waveform

In CC operation, the CC loop control function of AP3968/69/70 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

3

4

Toffs

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

Toffs

Tons

Tons

Ipks

2

1

Iout









…………(7)