Embedded Systems Design - July 2008 - (Page 33)

feature Efficiency at nominal V IN, with different loads. VIN (V) IIN (mA) 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 Table 3 VO (V) 3.612 3.612 3.612 3.612 3.612 3.612 3.612 3.612 3.612 3.612 3.612 IO (A) 60µ 0.14 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 Efficiency (%) 00.00% 75.25% 77.18% 78.52% 79.21% 80.48% 82.84% 82.63% 82.47% 79.21% 76.98% 5.78 56.00 75.00 106.00 136.00 164.00 192.00 227.00 262.00 312.00 391.00 vider is fixed at 24 µA, to ensure that the reference current (0.5 µA, with thermal drifts) doesn’t have a significant effect on the output voltage. Also, the signal delay due to input capacitance is not enough to warrant the substitution of a capacitive divider. PWM controller The classic UC3845 shown in Figure 3 draws about 17 mA (VFB and VSENSE = 0 V), which is excessive for this application. One possible alternative is the MAX5021 currentmode PWM controller. Available in a SOT23-6 package, it is among the smallest in its class. It also has the lowest typical current draw (1.2 mA), plus a 260kHz internal oscillator, VISENSE of 0.6 V, direct input from the optocoupler, and other features suitable for this application. One drawback, however, is an undervoltage-lockout threshold of 10 Voff/24 Von, which makes it also unsuitable for this particular 12-V input application. On the other hand, its ultra-low standby current makes it the preferred choice for other, higher-input-voltage applications. The final IC to be considered is the UCC38C41, which specifies an undervoltage lockout of 6.6 Voff/7.0 Von and a typical current draw of ICC = 2.3 mA. In the voltage adder, the current detector draws 100 µA (ICS = 2 µA), and the phototransistor of the optocoupler, 530 µA. To allow that level of phototransistor current, the LED must draw somewhat more than 1 mA. The resulting power supply, shown in Figure 4, measures less than 50 x 30 mm. It includes one optocoupler for the control-loop feedback and one for measuring battery voltage at the input. The power-supply characteristics are: • • • Maximum output current = 1 A. Regulated output voltage = 3.6 V. No-load current draw = 5.7 mA. MEASUREMENTS AND RESULTS The Figure 3 prototype circuit included several wireless modules featuring discontinuous transmission, with currents reaching maximum peaks of 3A and a maximum average of approximately 1A. To reduce the current peaks and the associated radio problems, you should make use of the Can CMX Really Put TCP/IP On My Little Ole 8-bit Chip? Yes, Ma’am, CMX has been doing amazing things with RTOSes and TCP/IP stacks for many years now. If you haven’t visited us in a while, you are missing a lot of cool, new technology that is economical, royalty free, and comes with source code. It might just put the sparkle back in your eyes! • • • • • • • • Power = 3.6 W. Input voltage range: 10 V to 15 V. Nominal Vin = 12 V. Isolated (needs galvanic isolation). Step-down flyback topology. Voltage and current control loops. PWM control scheme. Switching frequency: fC = 250 kHz. CMX RTOSes and TCP/IP Stacks Support Most 8-, 16-, 32-bit Processors and DSP’s. 12276 San Jose Blvd Jacksonville, FL 32223 Ph: 904.880.1840 Fax: 904.880.1632 email: cmx@cmx.com www.cmx.com www.embedded.com | embedded systems design | JULY 2008 33 http://www.cmx.com http://www.cmx.com http://www.embedded.com

Table of Contents Feed for the Digital Edition of Embedded Systems Design - July 2008

Embedded Systems Design - July 2008 Contents #Include Parity Bit Programming Pointers Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design Building a Power Supply for Discontinuous Transmission Wireless Networks An Exception Primer Advertising Index Break Points Marketplace

Embedded Systems Design - July 2008

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