EM8565AS7 datasheet(9/12 Pages) EXCELLIANCE

9 / 12 page

2013/2/4

Rev.A.4

EM8565A

Functional Description

UVLO

An UVLO comparator is implemented in EM8565A

to monitor the VCC pin voltage. As shown in Fig. 13,

a hysteresis is built in to prevent the shutdown

from the voltage drop during startup. The UVLO

(on) and UVLO (off) are setting at 16V and 10V,

respectively.

IVCC

VCC

Start up Current (45uA)

Operation Current (2mA)

UVLO(on)

UVLO(off)

IHV current

HV Current Source

(1mA)

HV Current Source

shutdown (<1uA)

Fig. 13

Startup Operation

Fig. 14 shows a typical HV startup circuit and

transformer auxiliary winding for the EM8565A

application, it consumes only startup current

(typical 45uA) and the startup current drawn from

the HV pin to charge the VCC capacitor (CVCC).

When VCC reaches UVLO (on) voltage, EM8565A

begins switching and the HV startup current

switches off. Then, the power required is supplied

from the transformer auxiliary winding.

The

hysteresis of UVLO (off) provides more holdup time,

which allows using a small capacitor for VCC.

Fig. 14

Switching Frequency

To guarantee accurate frequency, EM8565A is

trimmed to 10% tolerance. The internal oscillator

also generates slope compensation, 75% maximum

duty limit.

Leading Edge Blanking (LEB)

Each time the power MOSFET turn on, the MOSFET

COSS, secondary rectifier reverse recovery current

and gate driver sourcing current comprise the

current spike. To avoid premature termination of

the switching pulse, a leading edge blanking time is

built in. During the blanking time (400nS), the

PWM comparator is off and cannot switch off the

gate driver.

Fig. 15

INPUT

GND

CVCC

HV current source

HV MOS

EM8565A

GND

VCC

CVCC

HV curre nt source

HV MOS

EM8565A

300 nS LEB

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