Micrel Inc.

MIC4801

January 2011

7

M9999-011811-A

VIN

is 3V to 5.5V. A minimum bypass capacitor of 2.2µF

should be placed close to the 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 MIC4801. It can also be used for dimming

by applying a PWM signal. See the PWM Dimming

Interface in the Application Information section for

details. Pulling the EN low for more than 40ms puts the

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 than 60µs to wake the part up

in a known state. This equates to a 8.3kHz 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.

RSET

theoretical average LED current can be estimated by

equation (1):

(1)

(2)

D is the duty cycle of the LED current during PWM

dimming. When the device is fully ON the duty cycle

in Figure 2.

Due to DC losses across current paths internal and

equation is modified by a factor K, where K is calculated

to be 0.140kΩ.

(3)

(4)

The modified LED current equation is more accurate in

determining the actual LED current based on the RSET

resistor value.

Peak LED Current

1

10

100

1000

1

10

100

1000

10000

RSET (kΩ)

D1

The D1 pins are the linear driver inputs for the LED.

Connect the anode of the LED to VIN and the cathode to

the D1 pins. All the D1 pins must be connected together.

The D1 voltage at dropout is the minimum voltage

required by the linear driver in order for the LED to be

fully biased.

GND

The ground pin is the ground path for the linear driver.

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.