MIC4802 Datasheet, PDF (8/12 Pages) Micrel Semiconductor – High Efficiency 800mA Single Channel Linear WLED Driver with Ultra Fast PWM™ Control

Micrel Inc.
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⁾.
(1) _{From the low IQ sleep mode higher PWM frequencies above 15kHz}
require a logic high enable signal for 60μs to first enable the MIC4802
prior to PWM dimming.
LED Current
vs. PWM Duty Cycle
900
VIN = 5V
800
700
f_PWM = 1kHz
600
500
f_PWM = 5kHz
400
300
200
f_PWM = 10kHz
100
0
0
20
40
60
80 100
DUTY CYCLE (%)
Figure 3. Average Current per LED Dimming
by Changing PWM Duty Cycle for PWM Frequencies
up to 20kHz
MIC4802
LED Current
vs. PWM Duty Cycle
900
800
f_PWM = 20kHz
700
600
f_PWM = 100kHz
500
400 ^{fPWM = 200kHz}
300
200
100
0
0
f_PWM = 500kHz
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
35
30
25
20
15
10
5
0
100
1000 10000 100000 1000000
FREQUENCY (Hz)
Figure 5. Minimum Duty Cycle
for Varying PWM Frequency
January 2011
Figure 6. PWM Signal at 1% Duty Cycle (I_avg = 8mA)
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