LD7832GS datasheet(9/16 Pages) LEADTREND

9 / 16 page

LD7832

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7832-DS-00 April 2013

Application Information

Operation Overview

LD7832 is an LED controller for lighting application. The

transition mode (TM) technique meets the requirement

of high power factor in high efficiency. It minimizes the

external component counts and the PCB size for

compact application.

LD7832 is also designed to operate in voltage-mode TM

mode. 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. LD7832 also

features LED open protection, LED short protection,

over current protection, CS short protection, open

feedback protection, and over temperature protection.

No extra mains voltage sensing is required as what the

traditional current mode PFC controllers behaves for

power saving.

Internal High-Voltage Startup Circuit and

Under Voltage Lockout (UVLO)

The traditional circuit provides the startup current

through a resistor to power up the PWM controller.

However, it consumes too significant power to meet the

current power saving requirement. In most cases, larger

resistor carries larger resistance; it wastes more time to

startup.

To achieve the optimized topology, as shown in Fig.1,

LD7832 is implemented with a high-voltage startup

circuit on HV pin for it. During startup, a high-voltage

current source sinks current from the output of

full-bridge rectifier to provide the startup current and

charge the VCC capacitor. Once VCC drops below PDR,

the charge current will remain at 1mA to protect the

circuit from being damaged, even in case VCC pin is

shorted to ground. In contrast, the charge current will

increase to 3mA once VCC rises above PDR voltage

threshold during start-up. Meanwhile, the VCC supply

current is as low as 280

A that most of the HV current is

reserved to charge the VCC capacitor. By using such

configuration, the turn-on delay time will be almost same

no matter under low-line or high-line conditions.

As the VCC voltage rises higher than UVLO(on) to

power the LD7832 and further to deliver the gate drive

signal, the high-voltage current source will be disabled

and the supply current is provided from the inductor.

Therefore, it would reduce the power loss on the startup

circuit and perform highly power saving.

An UVLO comparator is embedded to detect the voltage

on the VCC pin to ensure the supply voltage enough to

power on the LD7832 controller and in addition to drive

the power MOSFET.

As shown in Fig. 2, a hysteresis

is provided to prevent the shutdown from the voltage dip

during startup.

Vcc

UVLO(on)

UVLO(off)

HV Current

1mA

Startup Current

Vcc current

~ 0mA (off)

Operating Current

(Supply from Inductor)

PDR

3mA

Fig. 2

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