July 2009 8 M9999-070809
MIC2292/93 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 on EN pin as shown in Figure 1.
The MIC2292/93 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-efciency
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
MIC2292/93 through a series resistor as shown in
Figure 2. The LED current is decreased proportion-
ally 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 volt-
age 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.
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
V
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 MIC2292/93 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 MIC2292/93 has
two product options in the 8-lead MLF
®
with overvoltage
protection, OVP. The extra pins of the 8-leadMLF
®
package
allow the use of a dedicated OVP monitor with options for 15V
or 34V (see Figure 3). The reason for the two OVP levels
is to let users choose the suitable level of OVP for their ap-
plication. For example, a 4-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 specied limits.
VIN
EN
GND
SW
FB
OUT
V
IN
Figure 3. MLF
®
Package OVP Circuit
Start-Up and Inrush Current
During start-up, inrush current of approximately double the
nominal current ows 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, C
ss
, 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
V
IN
Figure 4. Soft-Start Circuit
