August, 2004

7

M9999-081104

MIC2291

Micrel

the inductor and the MIC2291, with short traces for good

noise performance.

Feedback Resistors

The MIC2291 utilizes a feedback pin to compare the output

to an internal reference. The LED current is adjusted by

selecting the appropriate feedback resistor value. The de-

sired current can be calculated as follows;

Where V

Dimming Control

There are two techniques for dimming control. One is PWM

dimming, and the other is continuous dimming.

1. PWM dimming control is implemented by applying

a PWM signal on EN pin as shown in Figure 1. The

MIC2291 is turned on and off by the PWM signal.

With this method, the LEDs operate with either

zero or full current. The average LED current is

increased proportionally to the duty-cycle of the

PWM signal. This technique has high-efficiency

because the IC and the LEDs consume no current

during the off cycle of the PWM signal. Typical

PWM frequency should be between 100Hz and

10kHz.

PWM

VIN

EN

SW

FB

GND

Figure 3. PWM Dimming Method

VIN

EN

SW

FB

5.11k

49.9k

GND

DC

Equivalent

Figure 4. Continuous Dimming

R2

V

REF

LED

I

=

2. Continuous dimming control is implemented by

applying a DC control voltage to the FB pin of the

MIC2291 through a series resistor as shown in

Figure 2. The LED intensity (current) can be dy-

namically varied applying a DC voltage to the FB

pin. The DC voltage can come from a DAC signal,

or a filtered PWM signal. The advantage of this

approach is that a high frequency PWM signal

(>10kHz) can be used to control LED intensity.