SM8122A datasheet(9/13 Pages) NPC

Teilenummer	SM8122A

Bauteilbeschribung	White LED Driver IC

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Hersteller	NPC [Nippon Precision Circuits Inc]

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SM8122A

NIPPON PRECISION CIRCUITS INC.—9

Brightness adjustment using FB pin (PWM signal input)

When the brightness is controlled by PWM signal (VPWM × Duty), the LED current (ILED) is given by equa-

tion 4.

If the values R1 = 30

Ω, 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.

If the values R1 = 30

Ω, 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.

Taking the above diagram as an example, inserting the values R1 = 30

Ω, 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).

Brightness adjustment circuit using FB pin

(PWM signal input)

SW

VOUT

FB

SM8122

R1

30

Ω

SBD

R3

50k

Ω

R2

20k

Ω

VDD

CE

COUT

1.0

µF

CIN

4.7

µF

VIN

3.6V

LED

R4

50k

Ω

C1

0.1

µF

PWM signal

Duty [

%]

VPWM [V]

VSS

L

4.7

µH

PWM signal vs. LED current

0

5

10

15

20

0.0

0.5

1.0

1.5

2.0

2.5

3.0

VPWM

× Duty [V]

R2

× (VPWM × Duty − VFB)

R3 +R4

R1

ILED =

VFB

−

... (4)

30

ILED ==

=

20,000

× (3 × 0 − 0.5)

50,000 + 50,000

0.5

−

0.6

20mA

... (5)

30

ILED ==

=

20,000

× (3 × 1 − 0.5)

50,000 + 50,000

0.5

−

0

0mA

... (6)

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