LD5857

8

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5857-DS-00 April 2014

Application Information

Operation Overview

The LD5857 is designed for current-mode control power

converters. It features current-mode control, including

cycle-by cycle current limit and the simplified loop

compensation.

Output Drive Stage

An output stage of a CMOS buffer, with typical driving

capability of 1.2A/-1.3A, is incorporated to drive the

power MOSFET directly.

The output voltage is

clamped at 12V to protect the MOSFET gate even when

the VCC voltage rises over 12V.

Under Voltage Lockout (UVLO)

An UVLO comparator is implemented in it to detect the

voltage across the VCC pin. It would turn on the LD5857

as it detects there

’s enough supply voltage to drive the

power MOSFET.

As shown in Fig. 7, a hysteresis is

built in to prevent shutdown from the voltage dip during

start up.

The turn-on and turn-off threshold level are

set at 10.0V and 8V, respectively.

Vcc

UVLO(on)

UVLO(off)

t

t

I(Vcc)

Shutdown Current

(~uA)

Operating Current

(~ mA)

Fig. 7

LED Open Protection and OVP Trip Point

OVP voltage. Once it rises over the threshold of around

2.0V, the MOSFET will disable drive output (DRV).

B

A

B

OVP

R

R

R

2.0

V







signal ground as close as possible. It

’s superior to

suppress the noise and protect OVP from abnormal

condition.

Programming the LED Current

below parameter) to set the LED current.

LED

FB

I

V

3

.

0

R



Dimming Output of LED Current

The output current can be achieved by applying a PWM

signal to DIM pin, the brightness is adjusted in direct

proportion to the width of duty cycles. This frequency of

input signal varies in the range from 200Hz to 30KHz .

Also the LD5857 can program analog diming with

software

control.

The

dimming linearity

offset

is

approaching to 1%( typ.).

DIM Freq.=20KHz

DIM pin Duty Cycle

Fig8. The Curve between LED Current and DIM duty