LD7835

9

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7835-DS-01 July 2015

Application Information

Operation Overview

LD7835 is an LED controller for lighting application. The

transition mode (TM) technique meets the requirements

for high power factor and high efficiency. It minimizes

the external component counts and the PCB size for

compact application.

The control mechanism of LD7835 is a voltage-mode

operation. The switch turn-on time is fixed while the

turn-off time varies in steady state. Therefore, the switch

frequency changes in response to the different voltages.

The affected frequency reduces EMI noise. LD7835 also

features LED open protection, LED short protection,

over current protection, and over temperature protection.

No extra mains voltage sensing is required as what the

traditional current mode PFC controllers behaves for

power saving.

Ramp

Generator

Block,

Zero

Current

Detection (ZCD) and ZCD OVP

Fig. 14 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. 15.

The LD7835 features transition mode (TM) operation.

The zero current detection block circuit detects the ZCD

signal to turn on the MOSFET soon after the voltage

across ZCD falls to 0.6V and also the current through

the inductor reaches zero. Instead, if there’s no signal

detected within 160

s, the time-out will generate a

signal to turn on MOSFET to ensure the system operate

properly.

PWM Comparator

R

S

0.6V/1V

Ramp

Generator

1.2V

OCP

Comparator

COMP

Q

GM

Fmax

Limit

Time

Out

Vref

ZCD

FB

Max On

Limit

CS

Fig. 14

During the delay time as shown in Fig. 15, the junction

capacitor of the MOSFET resonates with the inductor

and

the

drain-source

voltage

(VDS)

decreases

accordingly. So, the MOSFET consumes less voltage

and the power dissipation will be minimized.

Inductor

Current

Inductor

Voltage

0A

0V

0V

Delay Time

Minimum

Voltage Turn-on

OUT

RAMP

ZCD

Voltage

0.6V

0

Fig. 15

Fig. 16 illustrates the operation of over voltage

protection, where ZCD divider resistors continue to

detect the output voltage as the gate switches in off

state and return the information to ZCD. If the output

voltage increases to trip the threshold, it will signal that

there may be some LED(s) open.

The LD7835 will