mic4812 Micrel Semiconductor, mic4812 Datasheet - Page 7

• Manufacturer Part Number: mic4812
• Manufacturer: Micrel Semiconductor

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
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
WLED current for each channel is set by the external
R
signal to the EN pin. The MIC4812 accommodates a
wide PWM frequency range as outlined in the application
information section.
Block Diagram
As shown in Figure 1, the MIC4812 consists of six
current mirrors set to copy a master current determined
by the R
designated control block for enabling and dimming the
WLEDs.
V
The input supply (V
drivers and the control circuitry. The V
is 3V to 5.5V. A bypass capacitor of 2.2µF 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).
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
60μs 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 60μs.
Micrel Inc.
January 2011
IN
SET
resistor. Dimming is controlled by applying a PWM
SET
resistor. The linear drivers have a
IN
) provides power to the linear
IN
operating range
7
R
The R
driver by connecting a R
average LED current can be calculated by equation (1):
R
A plot of I
D1-D6
The D1 through D6 pins are the linear driver inputs for
WLED 1 through 6, respectively. Connect the anodes of
the WLEDs to V
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
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.
SET
SET
[kΩ] = ( 820 / I
SET
LED
pin is used to set the peak current of the linear
Figure 2. Peak LED Current vs. R
versus R
1000
100
10
IN
or
1
1
and each cathode of the WLEDs to D1
LED
paralleled
SET
[mA] ) + 0.139
Peak LED Current
is shown in Figure 2.
10
vs. R
RSET (kΩ)
SET
SET
resistor to ground. The
100
for
M9999-010511-A
higher
1000
SET
MIC4812
current
(1)