LD5760GS datasheet(11/20 Pages) LEADTREND

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LD5760

11/05/2015

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5760-DS-02 November 2015

Application Information

Operation Overview

As long as the requirement for green power becomes a

trend and the power saving is getting more and more

important for the switching power supplies and switching

adaptors, the traditional PWM controllers are not able to

support such new requirements. Due to the cost and

size limit, the PWM controller designer is bound to

integrate with more functions to reduce the external part

counts. The LD5760 is ideal for these applications. Its

detailed features are described as below.

Internal High-Voltage Startup Circuit and

Under Voltage Lockout (UVLO)

The traditional circuit provides the startup current

through a startup resistor to power up the PWM

controller. However, it consumes much significant

power to meet the current power saving requirement. In

most cases, startup resistors carry larger resistance and

spend more time to start up.

To achieve the optimized topology, as shown in Fig. 12,

LD5760 is implemented with a high-voltage startup

circuit for such requirement. At startup, the high-voltage

current source sinks current of AC Line/or Neutral to

provide startup current and charge the capacitor C1

connected to VCC.

At the startup transient, the HV current will supply

around 2.8mA to VCC capacitor until this VCC voltage

reaches the UVLO threshold VCC. By using such

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

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

EMI

Filte

Input

OTP

GND

OUT

VCC

LD5760

Fig. 12

As VCC trips UVLO(OFF), HV pin will recharge VCC

capacitor till VCC voltage rises back to UVLO(ON) again.

Since then, HV pin would no longer charge the capacitor

and instead, send a gate drive signal to draw supply

current for VCC from the auxiliary winding of the

transformer. That minimizes the power loss on the

start-up circuit successfully.

An UVLO comparator is embedded to detect the voltage

across the VCC pin to ensure the supply voltage enough

to power on the LD5760 and in addition, to drive the

power MOSFET.

As shown in Fig. 13, a hysteresis is

provided to prevent shutdown from the voltage dip

during startup.

The turn-on and turn-off threshold level

are set at 16V and 7V, respectively.

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