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Micrel, Inc.
Output Capacitor
A 1μF or greater output capacitor is sufficient for most
designs. An X5R or X7R dielectric ceramic capacitors
are recommended for designs with the MIC2291. Y5V
V IN
MIC2291
values may be used, but to offset their tolerance over
temperature, more capacitance is required.
VIN
SW
Diode Selection
The MIC2291 requires an external diode for operation. A
schottkey diode is recommended for most applications
PWM
EN
GND
FB
due to their lower forward voltage drop and reverse
recovery time. Ensure the diode selected can deliver the
peak inductor cur-rent, the maximum output current and
the maximum reverse voltage is rated greater than the
output voltage.
Input Capacitor
A minimum 1μF ceramic capacitor is recommended for
designing with the MIC2291. Increasing input
capacitance will improve performance and greater noise
immunity on the source. The input capacitor should be
as close as possible to 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 desired current can be calculated as follows;
Figure 3. PWM Dimming Method
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
dynamically varied applying a DC voltage to the
FB pin. The DC voltage can come from a DAC
signal, or a ? ltered PWM signal. The advantage
of this approach is that a high frequency PWM
signal (>10kHz) can be used to control LED
intensity.
V IN
V REF
R2 =
I LED
Where VREF is equal to 95mV.
Dimming Control
VIN
EN
SW
FB
GND
5.11k
49.9k
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.
DC
Equivalent
Figure 4. Continuous Dimming
May 2007
8
M9999-051507
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