三合微科股½有限公司
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
The information in this document is subject to change without notice.
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SAMHOP Microelectronics Corp. All Rights Reserved.
台北縣新店市民權路100號7樓
7F,No.100,Min-Chyuan Road, Hsintien, Taipei Hsien, Taiwan, R.O.C.
TEL: 886-2-2218-3978/2820 FAX: 886-2-2218-3320
Email : info@samhop.com.tw
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 Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
APPLICATIONS
* Cellular phone
* PDAs, handheld computer
* Digital camera
* MP3 player
* GPS receiver
* DC to DC Converter
SW
GND
FB
1
2
3
4
SHDN
5
V
IN
SW
GND
FB
1
2
3
6
5
4
V
IN
GND
SHDN
SW
GND
FB
1
2
3
4
SHDN
5
V
IN
PIN ASSIGNMENTS (TOP VIEW)
SOT-25
SOT-26
TSOT-25
(TSOT-23-5)
PIN DESCRIPTIONS
NO.
1
2
Pin name
SW
GND
Function
Switch Pin. Connect inductor / diode here.
Minimize trace area at this pin to reduce EMI.
Ground Pin. Connect directly to local ground plane
Feedback Pin. Reference voltage is 95mV. Connect
cathode of lowest LED and resistor here. Calculate
resistor value according to the formula:
R
FB
=95mV/I
LED
Shutdown Pin. Connect to 1.5V or higher to enable
device; 0.4V or less to disable device.
Ground Pin. Connect to Pin 2 and local ground
plane.
Input supply Pin. Must be locally bypassed.
3
FB
4
5
6
SHDN
GND (SOT-26)
V
IN
(SOT-26)
MAXIMUM RATINGS
Characteristic
Input voltage (V
IN
)
SW voltage
FB voltage
SHDN voltage
Operating temperature range
Maximum junction temperature
Storage temperature range
Lead temperature (soldering, 10 sec)
Rating
10
36
10
10
- 40 ~ 85
125
- 65 ~ 150
300
Unit
V
V
V
V
o
C
V
V
V
Page 2
V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
ELECTRICAL CHARACTERISTICS
Characteristic
Minimum operating voltage
Maximum operating voltage
Feedback voltage
FB pin bias current
Supply current
(T
A
=25
o
C , V
IN
=3V, V
SHDN
= 3V, unless otherwise noted)
Condition
Min.
2.5
-
I
SW
=100mA, Duty cycle=66%
86
10
-
SHDN=0V
Switching frequency
Maximum duty cycle
Switch current limit
Switch V
CESAT
Switch leakage current
SHDN voltage high
SHDN voltage low
SHDN pin bias current
I
SW
=250mA
V
SW
=5V
-
0.8
85
-
-
-
1.5
-
-
Typ.
-
-
95
45
1.9
0.1
1.2
90
320
350
0.01
-
-
65
Max.
-
10
104
100
2.5
1.0
1.6
-
-
-
5
-
0.4
-
Unit
V
V
mV
nA
mA
μA
MHz
%
mA
mV
μA
V
V
μA
Note :
1. Absolute maximum ratings are those values beyond which the life of the device may be impaired.
2. The SM8237 is guatanteed to meet specification from 0
o
C to 70
o
C. Specification over the -40
o
C
to 85
o
C operating temperature range are assured by design, characterization and correlation with
statistical process controls.
Page 3
V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
TYPICAL PERFORMANCE CHARACTERISTICS
FUNCTIONAL BLOCK DIAGRAM
Figure 1. SM8237 function block diagram
Page 4
V.1.3 Aug 28,2007
