SM8121A データシートの表示（PDF） - Nippon Precision Circuits
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SM8121A Datasheet PDF : 19 Pages
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SM8121A
Brightness adjustment using FB pin (PWM signal input)
VIN
3.6V
L
10µH
CIN
4.7µF
SBD
COUT
1.0µF
SW
VDD
VSS
SM8121
LED
FB
CE
PWM signal
R3
50kΩ
R2
20kΩ
R1
30Ω
R4
C1
Duty [%]
50kΩ
0.1µF
VPWM [V]
Brightness adjustment circuit using FB pin
(PWM signal input)
20
15
10
5
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
VPWM × Duty [V]
PWM signal vs. LED current
When the brightness is controlled by PWM signal (VPWM × Duty), the LED current (ILED) is given by equa-
tion 4.
VFB − R2 × (VPWM × Duty − VFB)
R3 +R4
ILED =
R1
... (4)
If the values R1 = 30Ω, R2 = 20kΩ, R3 = 50kΩ, R4 = 50kΩ, VFB = 0.5V, VPWM = 3V, and Duty = 0% are
inserted in equation 4, the LED current ILED = 20mA, as shown in equation 5.
0.5 − 20,000 × (3 × 0 − 0.5)
ILED =
50,000 + 50,000 = 0.6 = 20mA
30
30
... (5)
If the values R1 = 30Ω, R2 = 20kΩ, R3 = 50kΩ, R4 = 50kΩ, VFB = 0.5V, VPWM = 3V, and Duty = 100% are
inserted in equation 4, the LED current ILED = 0mA, as shown in equation 6.
0.5 − 20,000 × (3 × 1 − 0.5)
ILED =
50,000 + 50,000 = 0 = 0mA
30
30
... (6)
Taking the above diagram as an example, inserting the values R1 = 30Ω, R2 = 20kΩ, R3 = 50kΩ, R4 = 50kΩ,
VFB = 0.5V, VPWM = 3V, and Duty = 0 to 100% into equation 4 gives the maximum LED current ILED of
20mA when Duty = 0% (equation 5) and the minimum LED current ILED of 0mA when Duty = 100% (equa-
tion 6).
NIPPON PRECISION CIRCUITS INC.—11
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