AP3785T datasheet(13/18 Pages) DIODES

13 / 18 page

AP3785T

Document number: DS37552 Rev. 4 - 2

13 of 18

www.diodes.com

September 2015

AP3785T

Operation Principle Description (Cont.)

If the CBC resistance (RCBC) and RFB1 are fixed, the current (ICBC) flowing through the CBC resistance (RCBC) also follows the changing of the

VLOAD:

CBC

LOAD

CBC



(15)

The voltage change (

V

AUX) of auxiliary winding (VAUX) is varied with the VLOAD:

LOAD

CBC

AUX











(16)

The VO changing (VO) which VAUX reflects is also changed with VLOAD:

LOAD

CBC

aux

AUX













(17)

So, the right cable compensation can be achieved by adjusting RCBC. The 43kΩ resistance can ensure about 350mV cable compensation from

no load to full load.

12. Protection

The AP3785T has multiple built-in fault protection functions: FB over voltage protection, VCC over voltage protection, output short circuit

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

temperature protection. The different fault conditions trigger the different protection modes.

Protection Mode

The AP3785T has three protection modes: Dmin, auto-recovery and fast auto-recovery, which the operation principles are different.

When FB down-resistance short protection is triggered, the AP3785T enters the Dmin mode whereby the AP3785T immediately shuts down and

keeps a duration time tOFF(MAX) to output a pulse for turning on the primary switch, which is used to detect whether the fault condition is removed

and provide VCC supply. If the fault condition is removed before VCC decreases below VOPR(MIN), the device will enter normal operation mode. If

not, the AP3785T will repeat another Dmin mode action. When VCC drops below VOPR(MIN), AP3785T will enter the restart process, and VCC

voltage changes between VTH(ST) and VOPR(MIN) until FB down-resistance short condition is removed.

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

protection are triggered, the device enters the auto-recovery mode. Once the AP3785T 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 (I

CC_OPR) to standby current (I

CC_NL). Until

VCC voltage drops to VOPR(MIN), the AP3785T 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.

When VCC over voltage protection, FB over voltage protection, CBC short to GND protection, internal over temperature and external over

temperature protection are activated, the device enters the fast auto-recovery mode. The only difference with auto-recovery is that the VCC to

discharge current is kept operating current and the system can fast restart.

12.1

Short Circuit Protection (SCP)

Short Circuit Protection (SCP) detection principle is similar to the normal output voltage feedback detection by sensing FB pin voltage. When the

detected FB pin voltage is below VFB(SCP) for a duration of about tSCP, the SCP is triggered. Then the AP3785T enters auto-recovery mode that

the IC immediately shuts down and then restarts, so that the VCC voltage changes between VTH_ST and UVLO threshold until VFB(SCP) condition

is removed.

As to the normal system startup, the time duration of FB pin voltage below VFB(SCP) should be less than tSCP to avoid entering SCP mode. But for

the output short condition or the output voltage below a certain level, the SCP mode will be triggered.

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