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SC600
POWER MANAGEMENT
Applications Information (Cont.)
Brightness Control with PWM Input
Brightness control using a PWM input can be achieved
with the application circuit below. Note that by connecting
CX8 to the PWM signal the switching frequency is changed
as the PWM signal changes. The charge pump operates
at 262kHz during the on-time when there is a demand for
current, and then changes to 32kHz when there is no load
current. This is done to save battery power by taking ad-
vantage of the lower 280 μ A quiescent current at 32kHz.
The ripple voltage seen at the output is reasonable for
LED applications, but the output capacitance can be in-
creased to reduce the ripple if required. The sum of the
LED currents for this circuit is 55mA during the on-time.
White LED Driver Circuit with PWM Brightness Control
U1
2 .7V to 6. 5V
3
VI N
VOUT
1
5.0 V
C in
1 uF
6
EN
C F1 +
C F1 -
2
9
C buck et1
1 uF
Cout
1uF
4
5
C D4
C X8
C F2 +
C F2 -
10
7
C buck et2
1 uF
10 0
10 0
10 0
GN D
8
PW M
Voltage Waveforms for LED Driver Circuit
500
Ripple Voltage vs. Input Voltage
Vout p-p
120
Battery Current vs. Input Voltage
for Various Duty Cycles
89.20%
450
400
Vin p-p
100
70.30%
50.20%
30.20%
350
300
250
200
150
80
60
40
100
20
50
0
2.8
3.3
3.8
4.3
4.8
5.3
5.8
0
2.8
3
3.2
3.4
3.6
3.8
4
Input Voltage [V]
Input Voltage [V]
? 2006 Semtech Corp.
9
www.semtech.com
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