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© 2006 Semtech Corp.

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POWER MANAGEMENT

SC600

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 280μA 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

Voltage Waveforms for LED Driver Circuit

Cin

1uF

2.7V to 6. 5V

Cbucket1

1uF

Cbucket2

1uF

VI N

EN

CD4

CX8

GN D

CF2-

CF2+

CF1-

CF1 +

VOUT

3

6

4

5

8

7

10

9

2

1

U1

Cout

1uF

PW M

10 0

10 0

100

5.0 V

Ripple Voltage vs. Input Voltage

0

50

100

150

200

250

300

350

400

450

500

2.8

3.3

3.8

4.3

4.8

5.3

5.8

Input Voltage [V]

Vout p-p

Vin p-p

Battery Current vs. Input Voltage

for Various Duty Cycles

0

20

40

60

80

100

120

2.83

3.23.43.63.84

Input Voltage [V]

89.20%

70.30%

50.20%

30.20%