CPC9909

6

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R03

2. Functional Description

Figure 1 Typical Application Circuit

2.1 Overview

The CPC9909 drives the LEDs via a minimum

off-time, peak-current-limited, pulse-frequency

modulation scheme. This control scheme is inherently

stable, and the driver can be operated above a 50%

duty cycle without any open-loop instability or

sub-harmonic oscillations. Since the switching

frequency depends on the LED load current, it results

in a high efficiency operation.

2.2 LED Driver Theory of Operation

The typical application circuit is as shown in Figure 1

When PWMD is high, the control circuit is enabled and

the gate driver turns on the external power MOSFET

(Q1), causing the inductor (L1) current to ramp up until

exceeds this threshold, the gate driver turns Q1 off.

Q1 remains off for the duration of the fixed minimum

off-time. While the switch is off, the inductor continues

to deliver the current to the load though the diode

(D1). When the off-time expires, Q1 turns on again

until the peak current limit is reached, and the process

repeats.

The peak current limit threshold is set by the external

also be set externally via the LD pin. The lower of

in the inductor.

A soft start function can be implemented by ramping

the desired rate. To utilize the soft start function,

capacitor from the LD pin to ground, as shown in

Figure 2 Soft-Start Circuit

CPC9909

Voltage

Regulator

Voltage

Reference

Q

TRIG

Minimum Off-Time

One Shot

S

R

Q

-

+

-

+

V

DD

V

IN

LD

PWMD

GND

CS

GATE

8 - 550V

V

DD

R

T

R

T

R

SENSE

Q1

D1

L1

Buck Configuration

C

VDD

CPC9909

V

IN

CS

GND

GATE

R

T

LD

V

DD

PWMD

51k

Ω

2.2k

Ω

0.1

μF