MIC2843 datasheet(9/15 Pages) MICREL

9 / 15 page

Micrel Inc.

MIC2843

November 2010

M9999-110810-B

Functional Description

The MIC2843 is a six-channel WLED driver. The WLED

driver is designed to maintain proper current regulation

with LED current accuracy of 1.5% while the typical

matching between the six channels to be 3% at room

temperature. The WLEDs are driven independently from

the input supply and will maintain regulation with a

dropout of 40mV at 20mA. The low dropout of the linear

drivers 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 resistor. If

dimming is desired the MIC2843 can dim via an Ultra

Fast PWM™ signal.

Block Diagram

As shown in Figure 1, the MIC2843 consists of six

current mirrors set to copy a master current determined

by RSET. The linear drivers have a designated control

block for enabling and dimming of the WLEDs. The

MIC2843 is controlled by the PWM control block that

receives PWM signals for dimming.

VIN

The input supply (VIN) provides power to the linear

drivers and the control circuitry. The VIN operating range

is 3V to 5.5V. Due to wire inductance a minimum 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).

END

The END pin is equivalent to the enable pin for the linear

drivers on the MIC2843. It can also be used for dimming

using a PWM signal. See the “PWM Dimming Interface”

in the Application Information section for details.

The first pulse on the END pin must be equal or greater

to 40µs to wake the part up in a known state. This

equates to a 12.5kHz 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

40µs.

RSET

The RSET pin is used by connecting a RSET resistor to

ground to set the peak current of the linear drivers. The

average LED current can be calculated by Equation 1

below:

ILED (mA) = 410 × D / RSET (kΩ)

Eq. 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 ILED versus RSET is shown

in Figure 2.

Figure 2. Peak LED Current vs. RSET

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 VIN and each cathode of the WLEDs to D1

through D6. The linear drivers are independent of each

other.

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

section for more details.

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