AP3987CT

Document number: DS41644 Rev. 2 - 2

10 of 15

www.diodes.com

June 2019

© Diodes Incorporated

AP3987CT

Operation Description (continued)

8.

Leading Edge Blanking

When the power switch is turned on, a turn-on spike will occur on the sense-resistor. To avoid false-termination of the switching pulse, a leading-

edge blanking (from power MOSFET on) is built in. During this blanking period, the current sense comparator is disabled and the gate

driver can’t

be switched off.

9.

Valley Turn-On

When the off time (tOFF) is lower than tVAL-ON, the AP3987CT power system can work with valley turn-on. It can reduce MOSFET switching on

power losses which is resulted from the equivalent output capacitance to achieve highest overall efficiency. At the same time, because of valley

turn-on the switching frequency has the random jitter feature, which will be benefit for conductive EMI performance. And valley turn-on can also

reduce the power switch turn-on spike current and then achieve the better radiative EMI performance.

10. Adjustable Line Compensation

Since there is a constant delay time from the CS pin voltage reaching the given VCS reference to the power switch turning off, the real primary

peak current value always has a gap with the ideal value. The gap value changes with different input line voltage, which is caused by different

current rising slope, and results in different system constant current value.

In order to eliminate the constant current deviation due to line voltage, the adjustable line compensation is introduced to the AP3987CT design.

The negative voltage of FB pin which is linear to the line voltage is added up to VCS reference by a certain proportion and creates an adjustable

compensation voltage to clear up the primary current gap, so that the excellent line regulation of output current will be achieved.

RFB1

RFB2

FB

PFM

VAUX

VSAMPLE

VCS

R1

R2

RM

Vx

RLC

Figure 5. Adjustable Line Compensation Circuit

LINE

FB

M

LC

LC

P

AUX

CS

V

R

R

R

R

N

N

R

R

R

V



















1

2

1

1

………(14)

So, the AP3987CT can change the line compensation capability by adjusting the upper resistor at FB pin (RFB1). Higher resistance means lower

line compensation capability.

11. Protection

The AP3987CT has various built-in single point fault protection features: FB over voltage protection, VCC over voltage protection, output short

circuit protection, FB open circuit protection, transformer saturation protection and current sense resistor fault (short or open) protection, over

temperature protection. The fault conditions to trigger these protections are different and protection modes to enter are different after the

protections are triggered.

11.1 Protection Mode

The AP3987CT has three protection modes: auto-recovery, fast-recovery and hold. The operation principles are different.

When VCC over voltage protection, FB over voltage protection, output short circuit protection are activated, the AP3987CT enters the auto-

recovery mode. Once the AP3987CT enters the auto recovery mode, the device shuts down immediately and

doesn’t signal any pulse, the VCC

static current is decreased from operating current ICC_OPR to standby current ICC_NL. Until VCC voltage drops to VOPR(MIN), the AP3987CT will

enter the restart process, and VCC voltage changes between VTH(ST) and VOPR(MIN) until the fault condition is removed. The slope of VCC

voltage to discharge is very small, and the time to drop to VOPR(MIN) is very long. It can decrease the average power dissipation at a fault

condition.