SM8121A Datasheet, PDF (7/19 Pages) Nippon Precision Circuits Inc – White LED Driver IC

OPERATION OVERVIEW
SM8121A
VIN
2.3 to 5.5V
CIN
4.7µF
Enable
Disable
L
10µH
SBD
SW
Buff
VDD
CE
VSS
PWM
COMP
FB
ERR
AMP
RAMP
GENERATOR
OSC
VREF
SOFT
START
COUT
LED
1.0µF
ZD
R1
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-
nected between FB and VSS sets the LED current I_LED, where the resistance R1 is given by the following
equation.
R1 = 0.5 / I_LED
FB VFB=0.5V
ILED=0.5/R1
R1=0.5/ILED
NIPPON PRECISION CIRCUITS INC.—7