Micrel, Inc. MIC2292/93C
September 2009 9
M9999-091509-A
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 MIC2292/93C 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 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 MIC2292/93C through a series resistor as
shown in Figure 2. The LED current is
decreased proportionally with the amplitude of
the control voltage. 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 filtered PWM signal. The
advantage of this approach is that a high
frequency PWM signal (>10kHz) can be used to
control LED intensity.
PWM
VIN
EN
SW
FB
OUT
GND
V
IN
Figure 1. PWM Dimming Method
VIN
EN
SW
FB
OUT
5.11k
49.9k
GND
DC
IN
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 MIC2292/93C 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 MIC2292/93C has three product options
in the 8-pin MLF
®
with overvoltage protection, OVP. The
extra pins of the 8-pin MLF
®
package allow the use of a
dedicated OVP monitor with options for 15V 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, and
accordingly, smallest size and lowest cost. The OVP will
clamp the output voltage to within the specified limits.
VIN
EN
GND
SW
FB
OUT
IN
Figure 3. MLF
®
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 4 could be implemented. The soft-start capacitor,
Css, provides over-drive to the FB pin at start-up,
resulting in gradual increase of switch duty cycle and
limited inrush current.
VIN
EN
10k
2200pF
SW
OUT
FB
C
SS
R
GND
IN
Figure 4. One of Soft-Start Circuit
