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SM8120

White LED Driver IC

Nippon Precision Circuits  

SM8120A

Brightness Adjustment using CE Pin

The LED average current can be adjusted by controlling the duty of a PWM signal input on the CE pin. When

CE goes from LOW to HIGH, the soft start function operates (with 500µs constant soft start time) and, there-

fore, the LED average current ratio for a given PWM signal duty falls with increasing PWM signal frequency.

Taking this into consideration, the recommended PWM control signal has a frequency range of 100 to 400Hz

with duty cycle range of 10 to 90%.

L

22µH

VIN

3.6V

CIN

4.7µF

PWM signal

SBD

SW

VDD

VSS

FB

CE

COUT

1.0µF

LED

R1

25Ω

20.0

100Hz

15.0

400Hz

10.0

5.0

1000Hz

1400Hz

0.0

0

10

20

30

40

50

60

70

80

90 100

PWM signal duty [%]

Brightness adjustment circuit using CE pin

PWM signal duty vs. LED average current

When adjusting the brightness using the CE pin, a ripple voltage synchronized to the PWM signal is generated

across the output capacitor COUT. The amplitude of the ripple voltage is determined by the number of LEDs

and their forward-bias voltage drop characteristics. If a ceramic capacitor is used for the output capacitor

COUT, an audible noise may be generated due to the ceramic capacitor’s piezoelectric effect. The audible noise

level depends on the ceramic capacitor (capacitance, bias dependency, withstand voltage etc.), LEDs (number,

forward-bias voltage drop etc.), and mounting board (thickness, mounting conditions etc.), and thus should be

verified under actual conditions.

Alternatively, a tantalum capacitor or film capacitor with low piezoelectric effect can be used as the output

capacitor COUT to minimize the noise level, or the brightness can be adjusted using the FB pin as described

earlier. The audible noise generated when using the CE pin is not an inherent phenomena of the SM8120A

device, but of the brightness adjustment method employed.

11.0V

8.1V

COUT

20mA

3.5V

3.5V

3.5V

0.5V

COUT

0mA

2.7V

2.7V

2.7V

0V

Output voltage with LEDs ON Output voltage with LEDs CE input signal and output ripple voltage

OFF

NIPPON PRECISION CIRCUITS INC.—9

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