MIC2842A Datasheet, PDF (7/14 Pages) Micrel Semiconductor – High Efficiency 4 Channel WLED Driver with DAM™ and Single Wire Digital Control

Micrel Inc.
Functional Description
The MIC2842A 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 via an external
R_SET resistor. If dimming is desired the MIC2842A is
controlled by a single-wire digital interface.
Block Diagram
As shown in Figure 1, the MIC2842A consists of 4
current mirrors set to copy a master current determined
by R_SET. The linear drivers have a designated control
block for enabling and dimming of the WLEDs. The
MIC2842A dimming is controlled by an internal Digital
Control Interface.
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 input (VIN) pin and the ground (GND)
pin. Refer to the layout recommendations section for
details on placing the input capacitor (C1).
DC
The DC pin is used to enable and control dimming of the
linear drivers on the MIC2842A. See the MIC2842A
Digital Dimming Interface in the Application Information
section for details. Pulling the DC pin low for more than
1260μs puts the MIC2842A into a low Iq sleep mode.
The DC pin cannot be left floating; a floating enable pin may
cause an indeterminate state on the outputs. A 200kΩ pull
down resistor is recommended.
MIC2842A
R_SET
The R_SET pin is used by connecting a R_SET resistor to
ground to set the peak current of the linear LED drivers.
The average LED current can be calculated by the
equation (1) below.
I_LED (mA) = 410 * ADC / R_SET (kΩ)
(1)
ADC is the average duty cycle of the LED current
controlled by the single-wire digital dimming. See Table
1 for ADC values. When the device is fully on the
average duty cycle equals 100% (ADC=1). A plot of I_LED
versus R_SET at 100% duty cycle 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. 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.
August 2009
7
M9999-080309-D