SM8121A

NIPPON PRECISION CIRCUITS INC.—7

OPERATION OVERVIEW

The SM8121A basic structure is a step-up DC/DC converter. The booster control employs Pulse Width Modu-

lation (PWM) which controls the pulse duty cycle (75% max.) at constant frequency (1.0MHz 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 SM8121A 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 duty is restricted to 50%.

Selecting the Current-setting Resistor (R1)

The SM8121A control stabilizes the voltage on pin FB (0.5V typ.). Hence, the current-setting resistor R1 con-

equation.

R1

R1

ZD

SBD

L

10

µH

4.7

µF

2.3 to 5.5V

CE

SW

VSS

VDD

FB

ERR

AMP

Buff

VREF

SOFT

START

OSC

PWM

COMP

RAMP

GENERATOR

1.0

µF

Enable

Disable

LED

=0.5/R1

=0.5V