AP3968 datasheet(8/15 Pages) DIODES

8 / 15 page

AP3968/69/70

Document number: DS36759 Rev. 2 - 2

8 of 15

www.diodes.com

December 2013

AP3968/69/70

A Product Line of

Diodes Incorporated

Operation Description (Cont.)

Where fSW is the switching frequency. When the peak current Ipk is constant, the output power depends on the switching frequency fSW.

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:





AUX







…………(5)

Where Vd is the diode forward drop voltage, NAUX is the turns of auxiliary winding, and NS is the turns of secondary winding.

See equation 5

VAUX

Portion of Tons

Tons

Figure 3. Auxiliary Voltage Waveform

The output voltage is different from the secondary voltage in a diode forward drop voltage Vd which depends on the current. If the secondary

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

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.

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

Toffs

Tons  ………… (6)

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

Toffs

Tons

Ipks

Iout







…………(7)

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