August 2004

7

M0510-081104

MIC2287

Micrel

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.

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.

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

approach 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

overvoltage 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 OVP for 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

implemented. The soft-start capacitor, C

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