SM8122A Datasheet, PDF (5/13 Pages) Nippon Precision Circuits Inc – White LED Driver IC

OPERATION OVERVIEW
SM8122A
VIN
2.3 to 5.5V
CIN
4.7µF
L
4.7µH
SBD
SW
VDD
Buff
PWM
COMP
VOUT
OVP
COMP
LED
ERR
AMP
FB
COUT
1.0µF
Enable
RAMP
GENERATOR
VREF
Disable
CE
VSS
OSC
SOFT
START
R1
The SM8122A basic structure is a step-up DC/DC converter. The booster control employs Pulse Width Modu-
lation (PWM) which controls the pulse duty cycle (85% max.) at constant frequency (2.0MHz 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 SM8122A 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.
NIPPON PRECISION CIRCUITS INC.—5