MIC2844A Datasheet, PDF (7/14 Pages) Micrel Semiconductor – High Efficiency 6 Channel WLED Driver with DAM™ and Single Wire Digital Control

Micrel Inc.
Functional Description
The MIC2844A is a six 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 six 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 MIC2844A is
controlled by a single-wire digital interface.
Block Diagram
As shown in Figure 1, the MIC2844A consists of six
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 of 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 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 MIC2844A. See the MIC2844A
Digital Dimming Interface in the Application Information
section for details. Pulling the DC pin low for more than
1260μs puts the MIC2844A 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.
MIC2844A
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-D6
The D1 through D6 pins are the linear driver for WLED 1
through 6, respectively. When operating with less than
six 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 D6 leaving
D5 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