AL1663_17 datasheet(8/15 Pages) DIODES

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AL1663/AL1663R

Document number: DS38466 Rev. 2 - 2

8 of 15

www.diodes.com

March 2017

AL1663/AL1663R

Application Information (Cont.)

Start-up

After AC supply is powered on, the capacitor Cvcc across VCC and GND pin will be charged up by BUS voltage through a start-up resistor RTH.

Once VCC reaches VCC_TH, the internal blocks start to work. VCC will be supplied by VBUS until the auxiliary winding of Flyback transformer could

supply enough energy to maintain VCC above VOPR_MIN. If VCC voltage is lower than VOPR_MIN switch will be turned off.

After VCC exceeds VCC_TH

, the drive block won’t start to switch on/off signals until VCOMP is over the initial voltage VCOMP_ST which can be

programmed by RCOMP. The formula is shown as below. Such design can program startup on time to reduce the startup time or reduce the output

overshoot current.

COMP









700

Where VCOMP_ST is the pre-charged voltage of COMP pin. RCOMP is shown as Figures 1 and 2.

Generally, a big capacitance of CCOMP is necessary to achieve high power factor and stabilize th

e system loop (1μF to 2μF is recommended). The

pre-charged voltage in start-up procedure can be programmed by RCOMP.

Protections

1. Output Open Protection (OVP)

The output voltage is reflected by the voltage on transformer’s auxiliary winding. Both FB pin and VCC pin of IC integrate over voltage protection

function. When there is a rapid line and load transient, the output voltage may exceed the regulated value. If VCC exceeds VCC_OVP or VFB

exceeds VFB_CV, the over voltage protection will be triggered, switch will be turned off and VCC will be discharged. Once VCC is below VOPR_MIN.

the IC will shut down and be powered on again by BUS voltage through start up resistor.

Thus, output over voltage depends on the minimum voltage between both OVP protections

’ limitation. It can be gotten by below formula.

OVP

CC OVP

FB CV

AUX

Min













Where VOVP is the output over voltage setting; R5 and R6 that is shown as Figure 1 divide reflected voltage.

NAUX is the turns of auxiliary wind; NS is turns of the secondary wind. VCC_OVP is OVP Voltage of VCC.

2. Output short protection (OSP)

When the output is shorted, the output voltage is clamped to zero. The output voltage of the auxiliary winding, which is proportional to the output

winding, will drop down too. Once VCC is below VOPR_MIN, the IC will shut down and power on again by the BUS voltage through the start up

resistor.

3. Over Current Protection (OCP)

The AL1663/AL1663R has a build-in cycle by cycle over current protection of primary inductor current. When CS pin voltage reaches the voltage

VCS_OCP, switch will be turned off until next switch period. The maximum peak current (IPEAK (MAX)) of the inductor can be calculated as below:





CS OCP

PEAK MAX



Where VCS_OCP means primary current clamp voltage that is 1.2V.

RCS is current sense resistor which is shown as Figures 1 and 2.

4. Thermal Fold-back Protection (TFP)

The AL1663/AL1663R has thermal fold-back function: it adopts self-adaptive control method which can prevent the system from breaking down

caused by over temperature. The overheating temperature is set at +145°C. When the temperature of the IC is higher than this point; the device

will decrease the voltage reference of the CS linearly till OTP happens. By this way, the device can limit

system’s input power at high ambient

temperature, preventing system’s temperature increases further.

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