LD5523TGL datasheet(9/15 Pages) LEADTREND

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LD5523T

04/08/2021

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5523T-DS-00 April 2021

Application Information

Operation Overview

The LD5523T is built in a multi-mode PWM controller.

The LD5523T operates a constant frequency to achieve

the CCM as heavy load. For demanding higher power

efficiency and power-saving in light load condition, the

LD5523T implements QR function to allow the valley

switching and accomplish zero voltage switching (ZVS).

Under different load conditions, LD5523T provides the

different solutions for achieving higher efficiency and

performance.

Under Voltage Lockout (UVLO)

An UVLO comparator is implemented in it to detect the

voltage on the VCC pin. It would assure the supply

voltage enough to turn on the LD5523T. As shown in Fig.

9, a hysteresis is built in to prevent the shutdown from

the voltage dip during startup. The turn-on and turn-off

threshold level are set at 16.5V and 6.3V, respectively.

Fig. 9

Startup Current and Startup Circuit

The typical startup circuit to generate the LD5523T VCC

is shown in Fig. 10. During the startup transient, the

VCC is lower than the UVLO threshold thus LD5523T

will not to turn on. Therefore, the current through R1 will

provide the startup current and to charge the capacitor

C1. Whenever the VCC voltage is high enough to turn

on the LD5523T the supply current is provided from the

auxiliary winding of the transformer.

Lower startup current requirement on the PWM

controller will help to increase the value of R1 and then

reduce the power consumption on R1. By using CMOS

process and the special circuit design, the maximum

startup current of LD5523T is only 1

A. If a higher

resistance value of R1 is chosen, it usually takes more

time to start up. To select the value of R1 and C1

carefully will optimize the power consumption and

startup time.

Fig. 10

QR Mode Detection

The transformer will be demagnetized after the main

power MOSFET turns off. A quasi resonant signal will

be detected from auxiliary winding by FB pin through the

external resister.

As soon as the current of the secondary side diode is

down

zero

during

MOSFET-off

period,

the

transformer’s core is demagnetized completely. VDS of

MOSFET will resonate in discontinuous current mode.

The resonance frequency (FQR) will be obtained as

below.

)

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