SM8121A Datasheet, PDF (12/19 Pages) Nippon Precision Circuits Inc – White LED Driver IC

SM8121A
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%.
VIN
3.6V
L
10µH
CIN
4.7µF
SBD
COUT
1.0µF
SW
VDD
VSS
SM8121
LED
FB
CE
20.0
15.0
10.0
5.0
100Hz
400Hz
1000Hz
1400Hz
PWM signal
R1
25Ω
0.0
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 100
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 C_OUT. 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
C_OUT, 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 C_OUT 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 SM8121A
device, but of the brightness adjustment method employed.
11.0V
COUT
20mA
3.5V
3.5V
3.5V
0.5V
8.1V
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.—12