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SM8120

White LED Driver IC

Nippon Precision Circuits  

OPERATION OVERVIEW

SM8120A

VIN

3.0 to 4.5V

CIN

4.7µF

Enable

Disable

L

22µH

SBD

SW

VDD

Buff

CE

VSS

QS

R

OSC

COMP

LED

FB

COUT

1.0µF

ZD

VREF

SOFT

START

R1

The SM8120A basic structure is a step-up DC/DC converter. The booster control employs Pulse Frequency

Modulation (PFM) which controls the frequency (500kHz max.) at constant SW-Tr ON time (1.2µs typ.). The

LED current is set by a current-setting resistor R1 connected between pins FB (with stable voltage of 0.5V

typ.) and VSS.

When the switching transistor SW-Tr is ON, energy is stored in the inductor L. When SW-Tr is rapidly

switched OFF, the energy stored in the inductor generates a voltage across the terminals of the inductor. The

induced voltage, after being added to the input voltage, turns ON the Schottky barrier diode SBD and the

stored energy is transferred to the output capacitor. This sequence of events continues repeatedly, boosting the

output voltage.

The SM8120A features a built-in soft-start function. The soft-start time is approximately 500µs from after the

chip enable input CE rising edge. During this interval, the maximum SW-Tr ON time is restricted to 0.6µs.

Selecting the Current-setting Resistor (R1)

The SM8120A control stabilizes the voltage on pin FB (0.5V typ.). Hence, the current-setting resistor R1 con-

nected between FB and VSS sets the LED current ILED, where the resistance R1 is given by the following

equation.

R1 = 0.5 / ILED

FB VFB=0.5V

ILED=0.5/R1

R1=0.5/ILED

NIPPON PRECISION CIRCUITS INC.—4

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