AL1672 datasheet(7/14 Pages) DIODES

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AL1672

Document number: DS39344 Rev. 1 - 2

7 of 14

www.diodes.com

July 2017

AL1672

Functional Description and Application Information

The AL1672 is a constant current high PF BUCK converter, targeting LED lighting applications. The device integrates 600V/4A high voltage

MOSFET, which will help to cost down the whole system. High power factor is achieved by constant on time operation. In order to reduce the

switching losses and improve EMI performance, quasi-resonant switching mode is applied. The AL1672 integrates multiple protections including

UVLO protection, VCC over voltage protection, output open voltage protection, over current protection, thermal fold-back protection and over

temperature protection. The AL1672 can support multiple dimming modes including analog dimming mode and PWM dimming.

OUT

VCC

DRAIN

GND

RTH

Input

VR1

DB1

COMP

CCOMP

RCS

DRAIN

APWM

RCOMP

PWM/DC

Signal

CVCC

Figure 1: AL1672 BUCK Application Circuit

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 R2.

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

----------------------------------------------- (1)

Where VCOMP_ST is the pre-charged voltage of COMP pin, RCOMP is shown as Figure 1.

Generally, a big capacitance of C5 is necessary to achieve high power factor and stabili

ze the system loop (1μF~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 protect

ions’ limitation. It can be gotten by below formula.

OVP

CC OVP

AUX

Min













------------------------------------------------ (2)

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