MXHV9910

R01

www.clare.com

5

2. Functional Description

Figure 1 Typical Application Circuit

2.1 Overview

The MXHV9910 is a high-efficiency, low cost, off-line

LED driver designed using Clare's state of the art

BCDMOS on SOI process. The driver can operate

versatile input supply voltage range enables this driver

to be used in a broad range of applications such as flat

panel display RGB backlighting, signage, decorative

LED lighting, and incandescent lamp replacement.

The MXHV9910 IC is configured in a buck converter

topology, which is a perfect choice for off-line and DC

applications driving multiple LEDs in series or parallel.

This method provides excellent efficiency and enables

a buck switcher design using a minimum number of

external components. An external current sense

resistor sets the peak current to the LED string. In

addition, LED dimming can be implemented by either

applying a DC control voltage to the LD pin, or by

applying a low frequency, pulse-width modulated

digital signal to the PWMD pin (typically 500 Hz).

2.2 LED Driver Theory of Operation

The gate driver pulse width mode (PWM) control

circuit is enabled by connecting the PWMD pin to the

internal clock turns on the gate driver and the external

power MOSFET, causing the inductor current to ramp

up the voltage across the current sense resistor

located at the CS pin. When the rising voltage at the

set threshold, the gate drive signal goes low and turns

off the external power MOSFET. Turning the power

MOSFET off causes the inductor current to decay until

the next rising edge of the clock, and the process

repeats.

The peak current threshold is set by comparing the

be overridden externally by applying a voltage less

thresholds limits the peak current in the inductor

A soft-start function can be implemented by slowly

ramping up the DC voltage at the LD pin from 0mV to

a level greater than 250mV. Figure 2 shows a typical

recommended soft-start circuit design.

Figure 2 Soft-Start RC Network

+

-

+

-

Voltage

Regulator

Voltage

Reference

PWM

Control

250mV

V

DD

V

IN

R

T

LD

GND

PWMD

GATE

CS

V

DD

6

8

1

7

5

3

4

2

8-450V

R

SENSE

OSC

MXHV9910

V

IN

CS

GND

GATE

R

T

LD

V

DD

PWMD

51k

Ω

2k

Ω

0.1

μF