LD7831 datasheet(10/17 Pages) LEADTREND

10 / 17 page

LD7831

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7831-DS-00 April 2014

Vcc

UVLO(on)

UVLO(off)

HV Current

1.4mA

Startup Current

Vcc current

~ 0mA (off)

Operating Current

(Supply from Auxiliary Winding)

PDR

3mA

Regulator

Fig. 11

Ramp Generator Block and Zero Current

Detection (ZCD)

Fig. 12 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. 13.

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.

The setting for maximum ON-time will be achieved

according to the transformer, lowest AC line voltage,

and maximum output power. A choice of optimum

resistor value would result in best performance.

It will shut down the driving output if COMP pin voltage

falls below the threshold of zero ON-time. 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 0.3V. As ZCD pin voltage drops to 0.3V, 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 above 0.4V. Once it drops below 0.3V, the zero

current detector will activate to turn on the MOSFET.

PWM

Comparator

OCP

ZCD

0.3V/0.4V

COMP

Start Timer

0.3V

Zero ON

Time

Zero ON Time

Ramp generator

0.85V

Comparator

Fmax limit

ZCD Blanking

ZCD

Turn-On

Turn-Off

RZ1

RZ2

Naux

Fig. 12

The 140

s timer generates a MOSFET turn-on signal if

the driver output drops to low level for more than 140

s

from the falling edge of the driver output. Fig. 13 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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