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Micrel Inc.
Functional Description
The MIC4812 is a six channel linear WLED driver. The
WLED driver is designed to maintain proper current
regulation with LED current accuracy of 1%, and typical
matching of 1% across the six channels. The WLEDs
MIC4812
R SET
The R SET pin is used to set the peak current of the linear
driver by connecting a R SET resistor to ground. The
average LED current can be calculated by equation (1):
are driven independently from the input supply and will
maintain regulation with a dropout of 190mV at 100mA.
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
R SET [k ? ] = ( 820 / I LED [mA] ) + 0.139
A plot of I LED versus R SET is shown in Figure 2.
(1)
WLED current for each channel is set by the external
R SET resistor. Dimming is controlled by applying a PWM
signal to the EN pin. The MIC4812 accommodates a
1000
Peak LED Current
vs. R SET
wide PWM frequency range as outlined in the application
information section.
100
Block Diagram
As shown in Figure 1, the MIC4812 consists of six
current mirrors set to copy a master current determined
10
by the R SET resistor. The linear drivers have a
designated control block for enabling and dimming the
WLEDs.
1
1
10 100
1000
V IN
RSET (k ? )
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 2.2μF should be
placed close to input (VIN) pin and the ground (GND)
D1-D6
Figure 2. Peak LED Current vs. R SET
pin. Refer to the layout recommendations section for
details on placing the input capacitor (C1).
EN
The EN pin is equivalent to the enable pin for the linear
drivers on the MIC4812. 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
EN low for more than 20ms puts the MIC4812 into a low
Iq sleep mode. The EN pin cannot be left floating; a floating
enable pin may cause an indeterminate state on the outputs.
The first pulse on the EN pin must be equal or greater to
60 μ s to wake the part up in a known state. This equates to
3.3Khz PWM signal at equal or greater than 50% duty cycle.
Higher PWM frequencies may be used but the first pulse
must be equal or greater than 60 μ s.
The D1 through D6 pins are the linear driver inputs for
WLED 1 through 6, respectively. Connect the anodes of
the WLEDs to V IN and each cathode of the WLEDs to D1
through D6. 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
combinations, 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.
April 2011
7
M9999-041419-B
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