SM8122A Datasheet, PDF (9/13 Pages) Nippon Precision Circuits Inc – White LED Driver IC

SM8122A
Brightness adjustment using FB pin (PWM signal input)
L
4.7µH
VIN
3.6V
CIN
4.7µF
SBD
SW
VDD
VOUT SM8122
VSS
FB
CE
COUT
1.0µF
LED
PWM signal
R3
50kΩ
R2
20kΩ
R1
30Ω
R4
C1
Duty [%]
50kΩ
0.1µF
VPWM [V]
20
15
10
5
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
VPWM × Duty [V]
Brightness adjustment circuit using FB pin
(PWM signal input)
PWM signal vs. LED current
When the brightness is controlled by PWM signal (V_PWM × Duty), the LED current (I_LED) 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, V_FB = 0.5V, V_PWM = 3V, and Duty = 0% are
inserted in equation 4, the LED current I_LED = 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, V_FB = 0.5V, V_PWM = 3V, and Duty = 100% are
inserted in equation 4, the LED current I_LED = 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,
V_FB = 0.5V, V_PWM = 3V, and Duty = 0 to 100% into equation 4 gives the maximum LED current I_LED of
20mA when Duty = 0% (equation 5) and the minimum LED current I_LED of 0mA when Duty = 100% (equa-
tion 6).
NIPPON PRECISION CIRCUITS INC.—9