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Micrel Inc.
Functional Description
The MIC2841A is a four channel linear WLED driver.
The WLED driver is designed to maintain proper current
regulation with LED current accuracy of 1.5%, and
typical matching of 1.5% across the four channels. The
WLEDs are driven independently from the input supply
and will maintain regulation with a dropout of 40mV at
20mA. The low dropout allows the WLEDs to be driven
directly from the battery voltage and eliminates the need
for large and inefficient charge pumps. The maximum
WLED current for each channel is set by the external
R SET resistor. Dimming is controlled by applying a PWM
signal to the END pin. The MIC2841A accommodates a
wide PWM frequency range as outlined in the application
information section.
Block Diagram
As shown in Figure 1, the MIC2841A consists of four
current mirrors set to copy a master current determined
by the R SET resistor. The linear drivers have a
designated control block for enabling and dimming the
WLEDs.
V IN
The input supply (V IN ) provides power to the linear
drivers and the control circuitry. The V IN operating range
is 3V to 5.5V. A bypass capacitor of 1μF should be
placed close to the input (VIN) pin and the ground (GND)
pin. Refer to the layout recommendations section for
details on placing the input capacitor (C1).
END
The END pin is equivalent to the enable pin for the linear
drivers on the MIC2841A. It can also be used for
dimming using a PWM signal. See the PWM Dimming
Interface in the Application Information section for
details. Pulling the END low for more than 24ms puts the
MIC2841A into a low Iq sleep mode. The END pin cannot
be left floating; a floating enable pin may cause an
indeterminate state on the outputs. A 200k ? pull down
resistor is recommended.
MIC2841A
R SET
The peak current of the linear drivers is set by
connecting a resistor from the R SET pin to ground. The
average LED current can be calculated by equation (1)
below:
I LED (mA) = 410 * D / R SET (k ? ) (1)
D is the duty cycle of the LED current during PWM
dimming. When the device is fully on the duty cycle
equals 100% (D = 1). A plot of I LED versus R SET is shown
in Figure 2.
Figure 2. Peak LED Current vs. R SET
D1-D4
The D1 through D4 pins are the linear driver inputs for
WLED 1 through 4, respectively. Connect the anodes of
the WLEDs to V IN and each cathode of the WLEDs to D1
through D4. When operating with less than four WLEDs,
leave the unused D pins unconnected. The linear drivers
are extremely versatile in that they may be used in any
combination for example D1 thru D4 leaving D3
unconnected, or paralleled for higher current
applications.
GND
The ground pin is the ground path for the linear drivers.
The current loop for the ground should be as small as
possible. The ground of the input capacitor should be
routed with low impedance traces to the GND pin and
made as short as possible. Refer to the layout
recommendations for more details.
June 2009
7
M9999-062909-C
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