MIC2289C_11 Datasheet, PDF (7/9 Pages) Micrel Semiconductor – White LED Driver Internal Schottky Diode and OVP

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 MIC2289C 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 MIC2289C 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.
V_IN
PWM
VIN SW
OUT
EN
FB
GND
Figure 1. PWM Dimming Method
V_IN
VIN SW
OUT
EN
FB
GND
5.11k
49.9k
MIC2289C
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 MIC2289C to switch with a
high duty-cycle, resulting in output overvoltage. This
would normally cause the SW pin voltage to exceed its
maximum voltage rating, possibly damaging the IC and
the external components. However, the MIC2289C has a
dedicated OVP monitor to limit the output voltage within
safe levels (see Figure 3).
V_IN
VIN SW
OUT
EN
FB
GND
Figure 3. Thin SOT-23 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.
V_IN
2200pF
C_SS
VIN SW
OUT
EN
FB
GND
R 10k
Figure 4. One of Soft-Start Circuit
DC
Equivalent
Figure 2. Continuous Dimming
August 2007
7
M9999-081507-B