Micrel, Inc.

MIC2289C

August 2007

7

M9999-081507-B

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 MIC2289C 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 frequency

should be between 100Hz and 10kHz.

2. Continuous dimming control is implemented by

applying a DC control voltage to the FB pin of

the MIC2289C through a series resistor as

shown in Figure 2. The LED current is

decreased proportionally with the amplitude of

the control voltage. The LED intensity (current)

can be dynamically 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.

PWM

VIN

EN

SW

FB

OUT

GND

Figure 1. PWM Dimming Method

VIN

EN

SW

FB

OUT

5.11k

49.9k

GND

DC

Equivalent

Figure 2. Continuous Dimming

Open-Circuit Protection

If the LEDs are disconnected from the circuit, or in case

an LED fails open, the sense resistor will pull the FB pin

to ground. This will cause the MIC2289C to switch with a

high duty-cycle, resulting in output overvoltage. This

would normally cause the SW pin voltage to exceed its

maximum voltage rating, possibly damaging the IC and

the external components. However, the MIC2289C has a

dedicated OVP monitor to limit the output voltage within

safe levels (see Figure 3).

VIN

EN

GND

SW

FB

OUT

Figure 3. Thin SOT-23 Package OVP Circuit

Start-Up and Inrush Current

During start-up, inrush current of approximately double

the nominal current flows to set up the inductor current

and the voltage on the output capacitor. If the inrush

current needs to be limited, a soft-start circuit similar to

Figure 4 could be implemented. The soft-start capacitor,

CSS, provides over-drive to the FB pin at start-up,

resulting in gradual increase of switch duty cycle and

limited inrush current.

VIN

EN

10k

2200pF

SW

OUT

FB

R

GND

Figure 4. One of Soft-Start Circuit