AP3790

Document number: DS37221 Rev. 3 - 2

8 of 17

www.diodes.com

October 2014

© Diodes Incorporated

AP3790

A Product Line of

Diodes Incorporated

0V

See equation 5

Figure 3. Auxiliary Voltage Waveform

The voltage detection point is at a constant delay time of the D1 on-time. The constant delay time is changed with the different primary peak

current. The CV loop control function of AP3790 then generates a D1 off-time to regulate the output voltage.

Constant Current Operation

The AP3790 can work in constant-current (CC) mode. Figure 2 shows the secondary current waveforms.

proportion is

4

4



OFFS

ONS

t

t

(6)

OFFS

ONS

ONS

PKS

OUT

t

t

t

I

I









2

1

(7)

As to tight coupled primary and secondary winding, the secondary peak current is

PK

S

P

PKS

I

N

N

I





(8)

Thus the output constant-current is given by:

PK

S

P

OFFS

ONS

ONS

PK

S

P

OUT

I

N

N

t

t

t

I

N

N

I

















8

2

2

1

(9)

Therefore, AP3790 can realize CC mode operation by constant primary peak current and fixed diode conduction duty cycle.

Multiple Segment Constant Peak Current

As to the original PFM PSR system, the switching frequency decreases with output current decreasing, which will encounter audible noise issue

since switching frequency decreases to audio frequency range, about less than 20kHz.

In order to avoid audible noise issue, AP3790 uses 3-segment constant primary peak current control method. At constant voltage mode, the