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| (R) SP6642/3 Application Note: High Current Output s 40mA Output Current for the SP6642/3 Single Cell to 2V to 5V Converter SP6642/3 40mA Output Application: Block Diagram s High Efficiency: 81% s Twice the Output Current as the Pin-to-Pin Compatible MAX1642/3 Single Cell 47H SP6642/3 DC/DC Converter 2 to 5V Pagers, Remotes, Medical Devices s 0.75V Start-up Voltage Description The SP6642/6643 devices are high-efficiency, low-power step-up DC-DC converters for +1V inputs ideal for single alkaline cell applications such as pagers, remote controls, pointing devices and other low-power portable products. This application note is for users like medical devices, emergency lighting and others who need twice the output current than the datasheet specified values for the SP6642/6643. Typically specified at 20mA for 3.3V out with a 1.2V input, the SP6642 can actually output double the current if a lower value of inductor is selected. SP6642/6643 Applications Schematic - 47H Inductor SP6642/6643APN/02 SP6642/6643 Application Note (c) Copyright 2000 Sipex Corporation 1 High Current Application Changing the inductor on the SP6642/3 from 100H to 47H will increase the peak inductor current by 2 times from about 250mA to about 500mA. This can be seen from the relationship of inductor current in the equation VL = Ldi/dt where VL is set by Vin and Vout, dt is the charge time set by the On-Time Constant, K as specified in the SP6642/3 datasheet. Since L is the only factor changing, for a decrease of L by 1/2, the peak current, di, will double. With a doubling of peak current, the output current the SP6642/3 can provide will double. The saturation current specified for the inductor needs to be greater then the peak current to avoid saturating the inductor, which would result in a loss in efficiency and could damage the inductor. Table 1 below lists the inductors recommended for their low DC resistance and sufficient saturation current rating. INDUCTANCE VENDOR/PART NO. (H) 47 100 Sumida CD54-470 Sumida CD54-101 INDUCTOR SPECIFICATION RESISTANCE Isat (mA) 0.37 720 0.7 520 Comparing efficiencies, the 47H is less efficient than the 100uH at the maximum output current by about 4%. This difference can be attributed to the difference in time constant of charge for the inductor (L/R) in the equation for charging current: Ich = Imax*(1-e-t/(L/R)) where Imax is the peak inductor current, t is the charging time, L/R is the time constant for charge in the inductor. Since L changes by 1/2 from 100H to 47H, the DC series resistance R needs to decrease by 1/2 for L/R to remain the same. But, the DC resistance is composed of SP6642/3 switch resistance and the DC resistance of the inductor. Even though inductor resistance reduces by about 1/2 (table 1) the SP6642/3 switch resistance remains the same and the total DC resistance R will not reduce by 1/2. From this you can see that to decrease inductor values to be even less than 47H, the efficiency will continue to decrease and peak current could rise to unacceptable levels. In summary, the SP6642/3 can obtain higher output currents by using a properly selected inductor value. The efficiency curves show double the output current allowing the SP6642/3 to be used in a wider variety of portable products including medical devices and emergency lighting. Table 1. Recommended Surface Mount Inductors High Current Performance The curves in figures 1-6 illustrate SP6642 efficiency Vs output current using the Sumida 47uH inductor CD54-470 and the Sumida 100uH inductor CD54-101 for 3 different output voltages (2.4V, 3.3V, 5.0V) and 4 different input voltages (0.85V, 1.0V, 1.2V, 1.6V). These figures cover the full range of input and output voltages and currents for a single cell step-up converter for 2-5V output. Comparing figures 1,3,5 for the 47uH inductor to figures 2,4,6 for the 100H inductor and you will see about double the output current for the 47H Vs the 100H. For example, for 5V out for Vin of 1.0 to 1.6V (typical single cell voltages), the 47H maximum output current is 30 to 50mA, while the 100H maximum output is 15 to 25mA. SP6642/6643APN/02 SP6642/6643 Application Note (c) Copyright 2000 Sipex Corporation 2 SP6642 Efficiency vs. Output Current 100 90 80 70 Efficiency (%) Efficiency (%) 100 90 80 70 60 50 40 30 20 10 Vin = 1.6V Vin = 1.2V Vin = 1.0V Vin = 0.85V 60 50 40 30 20 10 0 0.01 0.1 1 Output Current (mA) Vin = 1.6V Vin = 1.2V Vin = 1.0V Vin = 0.85V 10 100 0 0.01 0.1 1 Output Current (mA) 10 100 Figure 1. 2.4VOUT Sumida 47H Figure 2. 2.4VOUT Sumida 100H 100 90 80 100 90 80 Efficiency (%) Efficiency (%) 70 60 50 40 30 20 10 0 0.01 0.1 1 Output Current (mA) Vin = 1.6V Vin = 1.2V Vin = 1.0V Vin = 0.85V 70 60 50 40 30 20 10 Vin = 1.6V Vin = 1.2V Vin = 1.0V Vin = 0.85V 10 100 0 0.01 0.1 1 Output Current (mA) 10 100 Figure 3. 3.3VOUT Sumida 47H Figure 4. 3.3VOUT Sumida 100H 100 90 80 100 90 80 Efficiency (%) Efficiency (%) 70 60 50 40 30 20 10 0 0.01 0.1 1 Output Current (mA) 10 100 Vin = 1.6V Vin = 1.2V Vin = 1.0V Vin = 0.85V 70 60 50 40 30 20 10 0 0.01 0.1 1 10 Output Current (mA) 100 Vin = 1.6V Vin = 1.2V Vin = 1.0V Vin = 0.85V Figure 5. 5VOUT Sumida 47H Figure 6. 5.0VOUT Sumida 100H SP6642/6643APN/02 SP6642/6643 Application Note (c) Copyright 2000 Sipex Corporation 3 ORDERING INFORMATION Please consult the factory for pricing and availability on a Tape-On-Reel option. Corporation SIGNAL PROCESSING EXCELLENCE Sipex Corporation Headquarters and Sales Office 22 Linnell Circle Billerica, MA 01821 TEL: (978) 667-8700 FAX: (978) 670-9001 e-mail: sales@sipex.com Sales Office 233 South Hillview Drive Milpitas, CA 95035 TEL: (408) 934-7500 FAX: (408) 935-7600 Sipex Corporation reserves the right to make changes to any products described herein. Sipex does not assume any liability arising out of the application or use of any product or circuit described hereing; neither does it convey any license under its patent rights nor the rights of others. SP6642/6643APN/02 SP6642/6643 Application Note 4 Model SP6642EU SP6643EU SP6642UEB Temperature Range -40C to +85C -40C to +85C Package Types 8-pin SOIC 8-pin SOIC SP6642 Evaluation Board (c) Copyright 2000 Sipex Corporation |
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