July 2005

7

M9999-070105

MIC2287

Micrel, Inc.

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

MIC2287 is turned on and off by the PWM signal.

Withthismethod,theLEDsoperatewitheitherzero

orfullcurrent.TheaverageLEDcurrentisincreased

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.

2. Continuous dimming control is implemented by

applying a DC control voltage to the FB pin of the

MIC2287 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 ap-

proach is a high frequency PWM signal (>10kHz)

that can be used to control LED intensity.

PWM

VIN

EN

SW

FB

GND

Figure 1. PWM Dimming Method

VIN

EN

SW

FB

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 MIC2287 to switch with a high

duty-cycle, resulting in output overvoltage. This may cause

the SW pin voltage to exceed its maximum voltage rating,

possibly damaging the IC and the external components. To

ensure the highest level of protection, the MIC2287 has 3

product options in the 8-lead 2mm × 2mm MLF™ with over-

voltage protection (OVP). The extra pins of the 8-lead 2mm

×

2mm MLF™ package allow a dedicated OVP monitor with

options for 15V, 24V, or 34V (see Figure 3). The reason for

the three OVP levels is to let users choose the suitable level

of OVPfor their application. For example, a 3-LED application

would typically see an output voltage of no more than 12V, so

a 15V OVP option would offer a suitable level of protection.

This allows the user to select the output diode and capacitor

with the lowest voltage ratings, as well as smallest size and

lowest cost. The OVP will clamp the output voltage to within

the specified limits. For the Thin SOT-23-5 package, an OVP

pin is not available.An external zener diode can be connected

from the output of the converter to FB pin as shown in Figure

4. to implement similar protection.

VIN

EN

GND

SW

FB

OVP

Figure 3. MLF™ Package OVP Circuit

VIN

EN

SW

FB

GND

5.11k

Figure 4. 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 5 could be

drive to the FB pin at start-up, resulting in gradual increase

of switch duty cycle and limited inrush current.

VIN

EN

SW

FB

2200pF

R

10k

GND

Figure 5. Soft-Start Circuit