LD7830 datasheet(10/18 Pages) LEADTREND

10 / 18 page

LD7830

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7830-DS-01a

August 2012

Vcc

UVLO(on)

UVLO(off)

HV Current

1.3mA

Startup Current

Vcc current

~ 0mA (off)

Operating Current

(Supply from Auxiliary Winding)

PDR

3.1mA

Fig. 14

Ramp Generator Block and Zero Current

Detection (ZCD)

Fig. 15 shows typical ramp generator block and ZCD

block. The comp pin voltage and the output of the ramp

generator block are compared to determine the MOSFET

ON-time, as shown in Fig. 16.

A greater comp voltage produces more ON-time. Using

an external resistor connected to ZCD pin to set the

desired slope of the internal ramp, the user may program

the maximum ON-time. Alternatively, the ON-time will

also achieve its maximum when COMP pin voltage trip

OLP trigger point.

The maximum ON-time should be set according to the

condition of the transformer, lowest AC line voltage, and

maximum output power. A choice of optimum resistor

value would result in best performance.

It shuts down the drive output if COMP pin voltage falls

below zero ON-time threshold. This optimizes the

efficiency in power saving in most conditions.

The Zero Current Detection block will detect auxiliary

winding signal to drive MOSFET as ZCD pin voltage

drops to 0.1V. As ZCD pin voltage drop to 0.1V, the

current through the transformer is below zero. This

feature enables transition-mode operation. The ZCD

comparator would not operate if ZCD pin voltage remains

at above 0.2V. Once it drops below 0.1V, the zero current

detector will act to turn on the MOSFET.

PWM

Comparator

OCP

ZCD

0.1V/0.2V

COMP

Start Timer

0.8V

Zero ON

Time

Burst Mode

Ramp generator

0.5V

Comparator

Vref

Fmax limit

ZCD Blanking

ZCD

Turn-On

Turn-Off

RZ1

RZ2

Naux

Fig. 15

The 150

s timer generates a MOSFET turn-on signal if

the driver output drops to low level for more than 150

s

from the falling edge of the driver output. Fig. 16 shows

typical ZCD-related waveforms. Since ZCD pin carries

some capacitance, it produces some delay to the turn-on

time caused from Rz1. During delay time, the junction

capacitor of the MOSFET resonates with the primary

inductor of the transformer and the drain-source voltage

(VDS) decreases accordingly. So, the MOSFET consumes

less voltage to turn on and it therefore minimizes the

power dissipation.

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