EM8564AS7 datasheet(9/12 Pages) EXCELLIANCE

9 / 12 page

2013/05/16

A.1

EM8564A

Functional Description

UVLO

An UVLO comparator is implemented in EM8564A

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 EM8564A

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, EM8564A

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, EM8564A is

trimmed to 10% tolerance. The internal oscillator

also generates slope compensation, 75% maximum

duty limit.

Leading Edge Blanking (LEB)

Fig.15 shows an 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

(380nS), the PWM comparator is off and cannot

switch off the gate driver.

Fig. 15

INPUT

GND

CVCC

HV current source

HV MOS

EM8564A

GND

VCC

CVCC

HV curre nt source

HV MOS

EM8564A

380nS LEB

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