SM8120 Datasheet, PDF (4/14 Pages) Nippon Precision Circuits Inc – White LED Driver IC

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 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.—4