LD5850

8

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5850-DS-00

November 2012

Application Information

Operation Overview

The LD5850 is designed for current-mode control power

converters. It provides all the advantages of 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 a power

MOSFET directly.

The output voltage is clamped at 14V

to protect the MOSFET gate even when the VCC voltage

is higher than 14V.

Under Voltage Lockout (UVLO)

An UVLO comparator is implemented in it to detect the

voltage across the VCC pin. It would detect if the supply

voltage is enough to turn on the LD5850 and further to

drive the power MOSFET.

As shown in Fig. 1, a

hysteresis is built in to prevent the 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)

Standby Current

(~ mA)

Fig. 1

LED Open Protection and OVP Trip Point

OVP voltage. Once it rises up to the threshold of around

2.5V, the MOSFET drive output (DRV) will turn off and the

IC restart it only after OVP voltage drops below 1.5V.

B

A

B

OVP

R

R

R

2.5

V







signal ground as close as possible. It

’s superior to

suppress the noise and protect OVP from abnormal

condition.

Programming the LED Current

LED current.

LED

FB

I

V

3

.

0

R



Dimming Control

The LED brightness control is accomplished by the PWM

signal from DIM pin in proportion to the various duty

cycles. LD5850 can apply the external PWM signal to DIM

pin in the range from 100Hz to 1 KHz with a swing voltage

of 0V to a level greater than 2.0V.

Current

Sensing

and

Leading-edge

Blanking

The LD5850 detects the primary MOSFET current across

the CS pin for the protection of cycle-by-cycle current limit.

The voltage threshold of the current sensing pin is set at

0.5V maximum. The MOSFET peak current can be

obtained as below.

CS

)

MAX

(

PEAK

R

V

5

.

0

I



A 275ns leading-edge blanking (LEB) time is programed

in the input of CS pin to prevent the false-triggering from

the current spike. The R-C filter is eliminable in those low

power applications, features pulse width of the turn-on