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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 to the EN pin, as shown in Figure
1. The MIC3287 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 time 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 MIC3287 through a series resistor, as shown
in Figure 2. The LED intensity (current) can be
dynamically varied with the applied DC control
voltage. The DC voltage can come from a DAC
signal, or a filtered PWM signal. The advantage
of the filtered PWM signal approach is a high
frequency PWM signal (>10kHz) can be used to
control LED intensity (current).
MIC3287
Open-Circuit Protection
If the LEDs are disconnected from the circuit, or in the case
that an LED fails, the sense resistor will then pull the FB pin to
ground. This will cause the MIC3287 to switch with a high duty-
cycle, resulting in output over-voltage. This may cause the SW
pin voltage to exceed its maximum voltage rating, damaging
the IC and the external components. To ensure the highest
level of protection, the MIC3287 has buit-in 24V over-voltage
protection functionality in the TSOT-23-6 pin and 8-pin MLF?
package options. The OVP will clamp the output voltage to
within the specified limits.
Start-Up and Inrush Current
If the inrush current needs to be limited, a soft-start circuit, simi-
lar to Figure 3, could be implemented. The soft-start capacitor,
C SS , provides overdrive to the FB pin at start-up, resulting in
gradual increase of switch duty cycle and limited in rush current.
V IN
C SS
2200pF
VIN SW
EN FB
V IN
GND
R
10k
VIN
SW
Figure 3. Soft-Start Circuit
PWM
EN
FB
GND
Figure 1. PWM Dimming Method
V IN
VIN
SW
EN
GND
FB
5.11k
49.9k
DC?
Equivalent
Figure 2. Continuous Dimming
April 2010
7
M9999-042910
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