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 Page 1 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 Page 2 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. 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 SAMHOP Microelectronics Corp. 1.2MHz STEP-UP CONVERTER TYPICAL APPLICATION SM8237 SM8237 Page 5 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 Page 8 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. Page 9 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 Page 10 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 Page 12 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