SamHop ELJPC220KF 1.2mhz step-up converter Datasheet

三合微科股份有限公司
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
The information in this document is subject to change without notice.
c 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 : [email protected]
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
GENERAL DESCRIPTION
FEATURES
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.
* 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
L1
22 uH
Vin
3.3 V
C1
4.7 uf
5
4
SD
Vout
5V / 100 mA
C2
1 uf
R1
51 K
1
Vin
OFF ON
D1
SW
SM8237
FB
C1 , C2 : X7R Dielectric
3
D1 : Frontier electronics SS14
L1 : Frontier electronics CSS0218P-22ON-LFR
R2
1K
GND
2
EFFICIENCY vs. LOAD CURRENT
100
5.15
95
90
EFFICIENCY (%)
EFFICIENCY
Efficiency (%) (%)
OUTPUT
VOLTAGE
Output
Voltage (V) (V)
OUTPUT VOLTAGE vs. LOAD CURRENT
5.20
5.10
Vin = 3.3V
5.05
5.00
4.95
4.90
4.85
4.80
Vin = 3.3V
85
80
75
70
65
60
55
Vout = 5V
50
0
10 20 30 40 50 60 70 80 90 100 105
0
10 20
30 40 50 60 70 80
90 100 105
Output Current (mA)
OUTPUT
CURRINT (mA)
OutputCURRENT
Current (mA)(mA)
OUTPUT
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V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
PIN ASSIGNMENTS (TOP VIEW)
APPLICATIONS
* Cellular phone
* PDAs, handheld computer
* Digital camera
* MP3 player
SW
1
GND
2
FB
3
* GPS receiver
* DC to DC Converter
5
4
VIN
SW
1
6
VIN
GND
2
5
GND
FB
3
4
SHDN
SHDN
SOT-25
SOT-26
SW
1
GND
2
FB
3
5
VIN
4
SHDN
TSOT-25
(TSOT-23-5)
PIN DESCRIPTIONS
NO.
Pin name
Function
1
SW
2
GND
Ground Pin. Connect directly to local ground plane
3
FB
Feedback Pin. Reference voltage is 95mV. Connect
cathode of lowest LED and resistor here. Calculate
resistor value according to the formula:
RFB =95mV/ILED
4
SHDN
5
GND (SOT-26)
6
VIN (SOT-26)
Switch Pin. Connect inductor / diode here.
Minimize trace area at this pin to reduce EMI.
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.
MAXIMUM RATINGS
Characteristic
Rating
Unit
Input voltage (VIN)
10
V
SW voltage
36
V
FB voltage
10
V
SHDN voltage
10
V
Operating temperature range
Maximum junction temperature
Storage temperature range
Lead temperature (soldering, 10 sec)
- 40 ~ 85
o
C
125
V
- 65 ~ 150
V
300
V
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V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
ELECTRICAL CHARACTERISTICS
(TA =25oC , VIN =3V, VSHDN = 3V, unless otherwise noted)
Characteristic
Min.
Typ.
Max.
Unit
Minimum operating voltage
2.5
-
-
V
Maximum operating voltage
-
-
10
V
86
95
104
mV
10
45
100
nA
-
1.9
2.5
mA
-
0.1
1.0
μA
Switching frequency
0.8
1.2
1.6
MHz
Maximum duty cycle
85
90
-
%
-
320
-
mA
ISW =250mA
-
350
-
mV
VSW =5V
-
0.01
5
μA
SHDN voltage high
1.5
-
-
V
SHDN voltage low
-
-
0.4
V
SHDN pin bias current
-
65
-
μA
Feedback voltage
Condition
ISW =100mA, Duty cycle=66%
FB pin bias current
Supply current
SHDN=0V
Switch current limit
Switch VCESAT
Switch leakage current
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 oC to 70 oC. Specification over the -40oC
to 85oC operating temperature range are assured by design, characterization and correlation with
statistical process controls.
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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
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V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
TYPICAL APPLICATION
SM8237
SM8237
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V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
TYPICAL APPLICATION
SM8237
SM8237
Page 6
V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
TYPICAL APPLICATION
SM8237
SM8237
Page 7
V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
OPERATION
The SM8237 uses a constant frequency, current mode control scheme to provide excellent line and load
regulation. Operation can be best understood by referring to the block diagram in figure 1. At the start of each
oscillator cycle, the SR latch is set, which turns on the power switch Q1. A voltage proportional to the switch
current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal pf the PWM
comparator A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset turning off
the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an
amplified version of the difference between the feedback voltage and the reference voltage of 95mV. In this
manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error
amplifier's output increases, more current is delivered to the output; if it decreases, less current is delivered.
Minimum output current
The SM8237 can regulate three series LEDs connected at low output currents, down to approximately 4mA
from a 4.2V supply, without pulse skipping, using the same external components as specified for 15mA
operation. As current is further reduced, the device will begin skipping pulses. This will result in some low
frequency ripple, although the LED current remains regulated on an average basis down to zero.
APPLICATIONS INFORMATION
Inductor selection
A 22u H inductor is recommended for most SM8237 applications. Although small size and high efficiency are
major concerns, the inductor should have low core losses at 1.2MHz and low DCR (copper wire resistance).
Some inductors in this category with small size are listed in Table 1.
Capacitor selection
The small size of ceramic capacitors makes them ideal for SM8237 applications. X5R and X7R types are
recommended because they retain their capacitance over wider voltage and temperature ranges than other
types such as Y5V or Z5U. A 1μF input capacitor and a 0.22μF output capacitor are sufficient for most of
SM8237 applicaiotns.
Diode selection
Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for
SM8237 applications. The forward voltage drop of a Schottky diode represents the conduction losses in the
diode, while the diode capacitance (C T or C D ) represents the switching losses. For diode selection, both
forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher current ratings
usually have lower forward voltage drop and larger diode capacitance, which can cause significant switching
losses at the 1.2MHz switching frequency of the SM8237. A Schottky diode rated at 100mA to 200mA is
sufficient for most SM8237 applications.
Part number
DCR
(Ω)
LQH3C220
0.71
Current rating
(mA)
250
ELJPC220KF
4.0
160
Panasonic
CDRH3D16-220 0.53
350
Sumida
LB2012B220M
1.7
75
Taiyo Yuden
LEM2520-220
5.5
125
Taiyo Yuden
Table 1. Recommended inductors
Manufacturer
Murata
Voltage drop Diode
Manufacturer
capacitance (pF)
(V)
Part
number
Forward
current (mA)
CMDSH-3
100
0.58 at
100mA
7.0 at 10V
Central
CMDSH2-3
100
0.49 at
200mA
15 at 10V
Central
BAT54
200
0.53 at
100mA
10 at 25V
Zetex
Table 2. Recommended Schottky diodes
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V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
APPLICATIONS INFORMATION
LED current control
The LED current is controlled by the feedback resistor. The feedback reference is 95mV. The LED current is 95
mV/R1. In order to have accurate LED current, precision resistors are preferred (1% is recommended). The
formula and table for R1 selection are shown below.
(1) R1 = 95 mV / ILED
Table 1. R1 resistor value selection
ILED (mA)
R1 (Ω)
5
19.1
10
9.53
12
7.87
15
6.34
20
4.75
SM8237
SM8237
Figure 2. LED driver with open-circuit protection
Setting the output voltage for DC to DC Converter
Set the output voltage by selecting the resistive voltage divider ratio. Using 51 KΩ for the High-side resistor R1
of the voltage divider. Determine the Low-side resistor R2 according to the following formula:
R2 =
R1 · VFB
Vout - VFB
Where Vout is the output voltage for R1= 51 KΩ and VFB = 0.095 V, then R2 (KΩ) = 4.845 / Vout -0.095
Please see the typical application on page 1.
Open-circuit protection
In the cases of output circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback
voltage will be zero. The SM8237 will then switch at a high duty cycle resulting in a high output voltage, which
may cause the SW pin voltage to exceed its maximum 36V rating. A zener diode can be used at the output to
limit the voltage on the SW pin (figure 2). The zener voltage should be larger than the maximum forward voltage
of the LED string. The current rating zener should be larger than 0.1mA.
Dimming control
There are four different types of dimming control circuits:
1. Using a PWM signal to SHDN pin
With the PWM signal applied to the SHDN pin, the SM8237 is turned on or off by the PWM signal. The LEDs
operate at either zero or full current. The average LED current increases proportionally with the duty cycle of the
PWM signal. A 0% duty cycle will turn off the SM8237 and corresponds to zero full LED current. A 100% duty
cycle corresponds to full current. The typical frequency range of the PWM signal is 1kHz to 10kHz. The
magnitude of the PWM signal should be higher than the minimum SHDN voltage high.
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V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
APPLICATIONS INFORMATION
2. Using a DC voltage
For some applications. The preferred method of brightness control is a variable DC voltage to adjust the LED
current. The dimming control using a DC voltage is shown in figure 5. As the DC voltage increases, the voltage
drop on R2 increases and the voltage drop on R1 decreases. Thus, the LED current decreases. The selection
of R2 and R3 will make the current from the variable DC source much smaller than the LED current and much
larger than the FB pin bias curent. For VDC range from 0V to 2V, the selection of resistors in figure 3 gives
dimming control of LED current from 0mA to 15mA.
3. Using a filtered PWM signal
The filtered PWM signal can be considered as an adjustable DC voltage. It can be used to replace the variable
DC voltage source in dimming control. The circuit is shown in figure 6.
4. Using a logic signal
For applications that need to adjust the LED current in discrete steps, a logic signal can be used as shown in
figure 5. R1 sets the minimum LED current (when the NMOS is off). R INC sets how much the LED current
increases when the NMOS is turned on. The selection of R1 and RINC follows formula (1) and Table 1.
Start-up and inrush current
To achieve minimum start-up delay, no internal soft-start circuit is included in SM8237. When first turned on
without an external soft-start circuit, inrush current is about 200mA . If soft-start is desired, the recommended
circuit and the waveforms are shown in figure 6. If both soft-start and dimming are used, a 10kHz PWM signal
on SHDN is not recommended. Use a lower frequency or implement dimming through the FB pin as shown in
figure 3,4 or5 .
SM8237
SM8237
SM8237
Figure 3 Dimming control using a DC voltage
Figure 4. Dimming control using a filtered PWM control
SM8237
SM8237
Figure 5. Dimming control using a logic signal
Figure 6. Recommended soft-start circuit
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V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
PACKAGE
Unit : mm
SM8237 Surface Mount SOT-25
D
4
B
C
5
L
b
1
2
3
H
A1
A
e
SYMBOL
MIN.
MOM.
A
0.889
1.295
A1
0.000
0.152
B
1.397
1.803
b
0.356
0.559
C
2.591
2.997
D
2.692
3.099
e
0.838
1.041
H
0.080
0.254
L
0.300
0.610
Page 11
V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
PACKAGE
Unit : mm
SM8237 Surface Mount SOT-26
D
5
1
2
4
B
C
6
L
b
3
H
A1
A
e
SYMBOL
MIN.
MOM.
A
0.889
1.295
A1
0.000
0.152
B
1.397
1.803
b
0.250
0.560
C
2.591
2.997
D
2.692
3.099
e
0.838
1.041
H
0.080
0.254
L
0.300
0.610
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V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
PACKAGE
Unit : mm
SM8237 Surface Mount TSOT-25 (TSOT-23-5)
R
D
θ
E1
L1
4
E
5
L
b
1
2
3
c
e
L2
Gauge
plane
e1
A1
y
A
A2
θ2
4xθ1
SYMBOL
MIN.
MOM.
MOM.
A
0.75
-
0.90
A1
0.00
-
0.10
A2
0.70
0.75
0.80
b
0.35
-
0.51
c
0.10
-
0.25
D
2.80
2.90
3.00
E
2.60
2.80
3.00
E1
1.50
1.60
1.70
e
0.95 BSC
1.90 BSC
e1
L
0.37
-
-
0.60 REF
L1
L2
0.25 BSC
y
-
-
0.10
R
0.10
-
-
θ
0°
-
8°
θ1
7° Nom
θ2
5° Nom
Page 13
V.1.3 Aug 28,2007
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