MIC2289C Datasheet, PDF (7/10 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
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 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 MIC2289C has 3 product options in the
2mm × 2mm MLF^®-8 with overvoltage protection, OVP.
The extra pins of the 2mm × 2mm MLF^®-8 package
allow a dedicated OVP monitor with options for 15V,
24V, 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, therefore smallest size and lowest cost. The
OVP will clamp the output voltage to within the specified
limits. For the Thin SOT-23-5 package, an OVP pin is
not available. An external zener diode can be connected
from the output of the converter to FB pin as shown in
Figure 4 to implement similar protection.
V_IN
PWM
VIN SW
OUT
EN
FB
GND
Figure 1. PWM Dimming Method
V_IN
VIN SW
OUT
EN
FB
GND
Figure 3. MLF^® Package OVP Circuit
V_IN
VIN SW
OUT
EN
FB
GND
5.11k
49.9k
DC
Equivalent
Figure 2. Continuous Dimming
September 2006
VIN SW
EN
FB
GND
5.11k
Figure 4. Thin SOT-23 Package OVP Circuit
7
M9999-090706