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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. 1007 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 0.22F, 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 0.22F output capacitor * Low profile TSOT package
TYPICAL APPLICATION
Vin 3.3 V C1 4.7 uf L1 22 uH D1 R1 51 K
3
Vout 5V / 100 mA C2 1 uf
C1 , C2 : X7R Dielectric D1 : Frontier electronics SS14
5 Vin
1 SW FB
OFF ON
4
SD
SM8237
GND 2
R2 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 Vin = 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
Vin = 3.3V
EFFICIENCY (%) EFFICIENCY (%) Efficiency (%)
5.15
90 100 105
Output Current (mA) OUTPUT CURRENT (mA)
Output Current (mA) OUTPUT CURRINT (mA)
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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 VIN SW GND FB 1 2 3 6 5 4 VIN GND SHDN SW GND FB 1 2 3 4 SHDN 5 VIN
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: RFB =95mV/ILED 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) VIN (SOT-26)
MAXIMUM RATINGS
Characteristic Input voltage (VIN) 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
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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
(TA =25oC , VIN =3V, VSHDN = 3V, unless otherwise noted) Condition Min. 2.5 ISW =100mA, Duty cycle=66% 86 10 SHDN=0V Switching frequency Maximum duty cycle Switch current limit Switch VCESAT Switch leakage current SHDN voltage high SHDN voltage low SHDN pin bias current ISW =250mA VSW =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 A MHz % mA mV A V V A
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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SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
TYPICAL PERFORMANCE CHARACTERISTICS
FUNCTIONAL BLOCK DIAGRAM
Figure 1. SM8237 function block diagram
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SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
TYPICAL APPLICATION
SM8237
SM8237
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SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
TYPICAL APPLICATION
SM8237
SM8237
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SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
TYPICAL APPLICATION
SM8237
SM8237
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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 1F input capacitor and a 0.22F 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 (CT or CD ) 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 LQH3C220 ELJPC220KF LB2012B220M LEM2520-220 DCR () 0.71 4.0 1.7 5.5 Current rating (mA) 250 160 350 75 125 Manufacturer Murata Panasonic Sumida Taiyo Yuden Taiyo Yuden BAT54 200 Part number CMDSH-3 CMDSH2-3 Forward current (mA) 100 100 Voltage drop Diode Manufacturer capacitance (pF) (V) 0.58 at 100mA 0.49 at 200mA 0.53 at 100mA 7.0 at 10V 15 at 10V 10 at 25V Central Central Zetex
CDRH3D16-220 0.53
Table 1. Recommended inductors
Table 2. Recommended Schottky diodes
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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) 5 10 12 15 20 R1 () 19.1 9.53 7.87 6.34 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 K 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 K and VFB = 0.095 V, then R2 (K) = 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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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). RINC 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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SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
PACKAGE
SM8237 Surface Mount SOT-25
D b
5 4
Unit : mm
C
1
2
3
B
H
e
SYMBOL A A1 A1 B b C D e H L A
MIN. 0.889 0.000 1.397 0.356 2.591 2.692 0.838 0.080 0.300
MOM. 1.295 0.152 1.803 0.559 2.997 3.099 1.041 0.254 0.610
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V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
PACKAGE
SM8237 Surface Mount SOT-26
D b
6 5 4
Unit : mm
C
1
2
3
B
H
e
SYMBOL A A1 A1 B b C D e H L A
MIN. 0.889 0.000 1.397 0.250 2.591 2.692 0.838 0.080 0.300
MOM. 1.295 0.152 1.803 0.560 2.997 3.099 1.041 0.254 0.610
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V.1.3 Aug 28,2007
SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
PACKAGE
SM8237 Surface Mount TSOT-25 (TSOT-23-5)
D b L1
5 4
Unit : mm
R
L
E
1
2
3
E1
c L2 Gauge plane 2 MOM. 0.75 2.90 2.80 1.60 0.95 BSC 1.90 BSC 0.37 0.60 REF 0.25 BSC 0.10 0 7 Nom 5 Nom 0.10 8 -
e e1
A2
SYMBOL A A1 A1 4x1 A2 b c D E E1 e e1 L L1 L2 y R 1 2 A
MIN. 0.75 0.00 0.70 0.35 0.10 2.80 2.60 1.50
MOM. 0.90 0.10 0.80 0.51 0.25 3.00 3.00 1.70
y
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