MIC2291 Datasheet, PDF (7/9 Pages) Micrel Semiconductor – 1.2A PWM Boost Regulator Photo Flash LED Driver

MIC2291
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;
R2 = ^VREF
I_LED
Where V_REF is equal to 95mV.
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.
Micrel
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.
V_IN
VIN SW
EN
FB
GND
5.11k
49.9k
DC
Equivalent
Figure 4. Continuous Dimming
V_IN
PWM
VIN SW
EN
FB
GND
Figure 3. PWM Dimming Method
August, 2004
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M9999-081104