AP3981C

Document number: DS41619 Rev. 2 - 2

10 of 16

www.diodes.com

June 2019

© Diodes Incorporated

AP3981C

Operation Description (continued)

CS

CS

PK

R

V

I



(2)

The energy stored in the magnetizing inductance LM each cycle is therefore:

2

2

1

PK

M

I

L

Eg







(3)

So the power transferring from the input to the output is given by:

SW

PK

M

f

I

L

P









2

2

1

(4)

Where, the fSW is the switching frequency. When the peak current IPK is constant, the output power would depend on the switching frequency fSW.

2.

Constant Voltage Operation

The output voltage is proportional to the auxiliary winding voltage during tONS period indicated by Formula 5, this auxiliary winding voltage is

divided by resistors RFB1 and RFB2 (refer to Figure 5) before inputting to the FB pin. As Figure 2 illustrated, the AP3981C detects the FB voltage at

the end of tSAMPLE during tONS period, the detected voltage which reflects the output voltage is regulated to VFB of 2.4V with the help of the

constant voltage control block in the AP3981C. For system design, adjusting the ratio of RFB1 and RFB2 can get the target output voltage value.





Vd

Vo

N

N

V

S

AUX

AUX







(5)

0V

tONS

tSAMPLE

tSW

Figure 2. Auxiliary Voltage Waveform

3.

Constant Current Control

In the AP3981C, Formula 6 shows the related parameters that determine the output current. To get a constant output current, the VCS and

tONS/tSW is fixed in AP3981C during CC mode. Meanwhile, the reliable control logic is integrated within AP3981C to ensure the system swift

smoothly between CC mode and CV mode.

(6)

SW

ONS

SW

ONS

OUT

t

t

*

Rcs

Vcs

*

Ns

Np

*

2

1

t

t

*

Ipk

*

Ns

Np

*

2

1

I



