SM8122A Datasheet, PDF (8/13 Pages) Nippon Precision Circuits Inc – White LED Driver IC

SM8122A
BRIGHTNESS ADJUSTMENT
Brightness Adjustment using FB Pin
The LED brightness can be adjusted using an input DC control voltage connected through resistor R3 to the FB
pin. Alternatively, the brightness can be controlled by a PWM signal by adding a low-pass filter comprising
resistor R4 and capacitor C1. The PWM signal frequency range is determined by the low-pass filter coeffi-
cients. For example, the recommended values for resistor R4 (50kΩ) and capacitor C1 (0.1µF) provide a PWM
signal frequency range of 1kHz to 1MHz.
Brightness adjustment using FB pin (DC voltage input)
VIN
3.6V
L
4.7µH
CIN
4.7µF
DC Voltage
0 to 3V
SBD
COUT
1.0µF
SW
VDD
VOUT SM8122
VSS
LED
FB
CE
R2
20kΩ
R3
R1
100kΩ
30Ω
20
15
10
5
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
DC voltage [V]
Brightness adjustment circuit using FB pin
(DC voltage input)
DC voltage vs. LED current
When the brightness is controlled by DC voltage (V_DC) connected to resistor R3, the LED current (I_LED) is
given by equation 1.
VFB − ^{R2 × (VDC − VFB)}
R3
ILED =
R1
... ₍₁₎
If the values R1 = 30Ω, R2 = 20kΩ, R3 = 100kΩ, V_FB = 0.5V, and V_DC = 0V are inserted in equation 1, the
LED current I_LED = 20mA, as shown in equation 2.
0.5 − ^{20,000 × (0 − 0.5)}
ILED =
100,000
= ^0.6 = 20mA
30
30
... ₍₂₎
If the values R1 = 30Ω, R2 = 20kΩ, R3 = 100kΩ, V_FB = 0.5V, and V_DC = 3V are inserted in equation 1, the
LED current I_LED = 0mA, as shown in equation 3.
0.5 − ^{20,000 × (3 − 0.5)}
ILED =
100,000
= ⁰ = 0mA
30
30
... ₍₃₎
Taking the above diagram as an example, inserting the values R1 = 30Ω, R2 = 20kΩ, R3 = 100kΩ, V_FB = 0.5V,
and V_DC = 0 to 3V into equation 1 gives the maximum LED current I_LED of 20mA when V_DC = 0V (equation
2) and the minimum LED current I_LED of 0mA when V_DC = 3V (equation 3).
NIPPON PRECISION CIRCUITS INC.—8