AP3987CT

Document number: DS41644 Rev. 2 - 2

8 of 15

www.diodes.com

June 2019

© Diodes Incorporated

AP3987CT

Operation Description (continued)

The relationship between the output current and secondary peak current IPKS is given by:

OFFS

ONS

ONS

PKS

OUT

t

t

t

I

2

1

I









(3)

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

PK

S

P

PKS

I

N

N

I





(4)

Thus the output constant current is given by:

PK

S

P

OFFS

ONS

ONS

PK

S

P

OUT

I

N

N

8

2

t

t

t

I

N

N

2

1

I

















(5)

Therefore, AP3987CT can realize CC mode operation by constant primary peak current and fixed secondary side conduction duty cycle.

5.

Multiple Segment Peak Current

As to the original PFM PSR system, the switching frequency decreases with decreasing of output current, 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, AP3987CT uses 3-segment primary peak current control method at Constant Voltage (CV) mode, the

current sense threshold voltages are multiple segments with different loading, as shown in Figure 4, which are VCS_H for high load, varied VCS_M

for medium load and VCS_L for light load. In no load and ultra light load condition (NL mode), the current reference is also VCS_L. But the

operation in NL mode is different, which will be described in next section.

At Constant Current (CC) mode, the peak current is still VCS_H when VO>VFB(SCP). If not, the peak current is 2/3* VCS_H.

It can be seen from the following figure that with multiple segment peak current control. The AP3987CT power system can keep switching

frequency above 20kHz from light load to heavy load and guarantee the audible-noise-free performance, and the maximum load system

switching frequency is not less than 50kHz, and no more than 70kHz.

fSW

VCS

IO/IOMAX

100%

40%

8%

50/70kHz

20/28kHz

1/2.25*VCSMAX

VCSMAX

FM

AM

FM

Figure 4. Segment Peak Current and Operating Frequency at CV Mode