Embedded Systems Design Europe - August/September 2008 - (Page 28)

psu design Efficiency at nominal V , with different loads. V (V) I (mA) IN IN IN V (V) O I (A) O Efficiency (%) 00.00% 75.25% 77.18% 78.52% 79.21% 80.48% 82.84% 82.63% 82.47% 79.21% 76.98% 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 Table 3 5.78 56.00 75.00 106.00 136.00 164.00 192.00 227.00 262.00 312.00 391.00 3.612 3.612 3.612 3.612 3.612 3.612 3.612 3.612 3.612 3.612 3.612 60µ 0.14 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 No-load current for different input voltages. V (V) I (mA) IN IN V (V) O I (A) O 10.0 12.0 15.0 Table 2 6.12 5.78 5.44 3.612 3.612 3.612 0.06 0.06 0.06 mentioned above (a “large” reference voltage of 2.5 V). Another option for this example is the TLV431C shunt regulator. Available from several manufacturers, it also meets the requirement of V = 1.24 V, with 1% tolerance from 0°C to +70°C. Current through the output divider 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. REF PWM controller The classic UC3845 shown in Figure 3 draws about 17 mA (V and V = 0 V), which is excessive for this application. One possible alternative is the MAX5021 current-mode 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, V 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 FB SENSE ISENSE V /24 V , 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 V /7.0 V and a typical current draw of I = 2.3 mA. In the voltage adder, the current detector draws 100 µA (I = 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: off on off on CC CS • • • • • • • • • • • Power = 3.6 W. Input voltage range: 10 V to 15 V. Nominal V = 12 V. Isolated (needs galvanic isolation). Step-down ﬂyback topology. Voltage and current control loops. PWM control scheme. Switching frequency: f = 250 kHz. Maximum output current = 1 A. Regulated output voltage = 3.6 V. No-load current draw = 5.7 mA. in C MEASUREMENTS AND RESULTS The Figure 3 prototype circuit included several wireless modules featuring discontinuous transmission, with cur28 AUGUST – SEPTEMBER 2008 | embedded systems design europe | www.embedded.com/europe 024-025-026-027-028-029_ESDE.indd 28 28/08/08 11:59:27 http://www.embedded.com/europe

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Embedded Systems Design Europe - August/September 2008 Contents TI Overhauls DSP Lineup, Adds 15 Processors QNX Publishes Source Code for File System Congatec to Take on Proprietary Market Swiss Multicore Project Wins Microsoft Grant OpenCores Bundles Development Tool ARM Compiler Boosts Freescale i.MX31 LabVIEW Updated for Multicore and Wireless Cover Feature: Interactive Tool Supports Multiprocessor SoC Design Wanted: Benchmaking for Embedded VMM Hypervisors Graphical Design Empowers Spider Robots Building a Power Supply for Discontinuous Transmission Wireless Networks RTOS Selection & Best Practices Achieving Cache Coherence in a MIPS32 Multicore Design New Products Advertising Contacts

Embedded Systems Design Europe - August/September 2008

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