Micrel Inc.

MIC4802

January 2011

8

M9999-013111-B

Application Information

Ultra Fast PWM™ Dimming Interface

The MIC4802 supports a wide range of PWM control

signal frequencies from 200Hz to 500kHz. This

extremely wide range of control provides ultimate

flexibility for handheld applications using high frequency

PWM control signals.

WLED dimming is achieved by applying a pulse width

modulated (PWM) signal to the END pin. For PWM

frequencies between 200Hz – 10kHz the MIC4802

supports a duty cycle range from 1% to 100%, as shown

in Figure 3. The MIC4802 incorporates an internal

shutdown delay to ensure that the internal control

circuitry remains active during PWM dimming. This

feature prevents the possibility of backlight flickering

when using low frequency PWM control signals. The

MIC4802 also supports Ultra Fast PWM™ frequencies

from 20kHz to 500kHz. Due to input signal propagation

delay, PWM frequencies above 20kHz have a non-linear

relationship between the duty cycle and the average

LED current, as shown in Figure 4 and 5. Figures 6

through 9 show the WLED current response when a

PWM signal is applied to the END pin

(1)

require a logic high enable signal for 60μs to first enable the MIC4802

prior to PWM dimming

.

LED Current

vs. PWM Duty Cycle

0

100

200

300

400

500

600

700

800

900

0

2040

6080

100

DUTY CYCLE (%)

VIN = 5V

Figure 3. Average Current per LED Dimming

by Changing PWM Duty Cycle for PWM Frequencies

up to 20kHz

LED Current

vs. PWM Duty Cycle

0

100

200

300

400

500

600

700

800

900

0

20

40

60

80

100

DUTY CYCLE (%)

Figure 4. Channel Current Response to PWM Control

Signal Frequencies from 50kHz to 500kHz

Minimum Duty Cycle

vs. Frequency

0

5

10

15

20

25

30

35

100

1000

10000

100000 1000000

FREQUENCY (Hz)

Figure 5. Minimum Duty Cycle

for Varying PWM Frequency