LD7538

10

Leadtrend Technology Corporation

LD7538-DS-00

November 2012

CS/OVP

VCC

GND

LD7538

OUT

LEB time

AC

Line

RS

Fig. 15

A leading-edge blanking (LEB) time is included in the

input of CS pin to prevent the false-trigger from the current

spike.

Output Stage and Maximum Duty-Cycle

An output stage of a CMOS buffer, with typical

250/-500mA driving capability, is incorporated to drive a

power MOSFET directly.

And the maximum duty-cycle

of LD7538 is limited to 75% to avoid the transformer

saturation.

Voltage Feedback Loop

The voltage feedback signal is provided from the TL431 at

the secondary side through the photo-coupler to the

COMP pin of the LD7538.

Similar to UC3842, the

LD7538 would carry a diode voltage offset at the stage to

feed the voltage divider at the ratio of RA and RB, that is,

)

V

V

(

R

2

R

R

V

F

COMP

)

PWM

(

COMPARATOR











A pull-high resistor is embedded internally and can be

eliminated externally.

Oscillator and Switching Frequency

The LD7538 is implemented with Frequency Swapping

function which helps the power supply designers to both

optimize EMI performance and lower system cost. The

switching frequency substantially centers at 65KHz, and

swap between a range of ±4KHz.

Green-Mode Operation

By using the green-mode control, the switching frequency

can be reduced under the light load condition. This feature

helps to improve the efficiency in light load conditions.

The green-mode control is Leadtrend Technology

’s own

property. Fig. 16 shows the characteristics of the

switching frequency vs. the comp pin voltage (VCOMP)

On/Off Control

The LD7538 can be turned off by pulling COMP pin lower

than 1.4V.

The gate output pin of the LD7538 will be

disabled immediately under such condition. The off-mode

can be released when the pull-low signal is removed.

VCOMP(V)

65kHz

25kHz

2.4

2.1

Vcs-off

1.4

Fig. 16

Internal Slope Compensation

In the conventional applications, the problem of the

stability is a critical issue for current mode controlling,

when it operates over 50% duty-cycle. As UC384X, It

takes slope compensation from injecting the ramp signal

of the RT/CT pin through a coupling capacitor. It therefore

requires no extra design for the LD7538 since it has

integrated it already.