SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
GENERAL DESCRIPTION
The SM8237 is a step-up DC/DC converter
specifically designed to drive white LEDs with a
constant current. The device can drive two, three
or four LEDs in series from a Li-ion cell. Series
connection of the LEDs provides identical LED
currents resulting in uniform brightness and
eliminating the need for ballast resistors. The
SM8237 switches at 1.2 MHz, allowing the use of
tiny external components. The output capacitor can
be as small as 0.22μF, saving space and cost
versus alternative solutions. A low 95mV feedback
voltage minimizes power loss in the current setting
resistor for better efficiency.
FEATURES
* Inherently matched LED current
* High efficiency : 84% typical
* Drives up to four LEDs from a 2.8V supply
* Drives up to six LEDs from a 5V supply
* 36V rugged bipolar switch
* Fast 1.2MHz switching frequency
* Uses tiny 1 mm tall inductors
* Requires only 0.22μF output capacitor
* Low profile TSOT package
TYPICAL APPLICATION
V
in
3.3 V
C
1
4.7 uf
L
1
22 uH
D
1
R
1
51 K
3
V
out
5V / 100 mA
C
2
1 uf
C1 , C2 : X7R Dielectric
D1 : Frontier electronics SS14
5
V
in
1
SW
FB
OFF ON
4
SD
SM8237
GND
2
R
2
1K
L1 : Frontier electronics CSS0218P-22ON-LFR
OUTPUT VOLTAGE vs. LOAD CURRENT
EFFICIENCY vs. LOAD CURRENT
100
95
90
85
80
75
70
65
60
55
50
0
10 20
30 40 50 60 70 80
Vout = 5V
V
in
= 3.3V
OUTPUT VOLTAGE (V)
Output Voltage (V)
5.20
5.10
5.05
5.00
4.95
4.90
4.85
4.80
0
10 20 30 40 50 60 70 80 90 100 105
V
in
= 3.3V
EFFICIENCY (%)
EFFICIENCY (%)
Efficiency (%)
5.15
90 100 105
Output Current (mA)
OUTPUT CURRENT (mA)
Output Current (mA)
OUTPUT CURRINT (mA)
Page 1
V.1.3 Aug 28,2007
SAMHOP [ SAMHOP MICROELECTRONICS CORP. ]
