Embedded Systems Design Europe - August/September 2008 - (Page 27)

psu design Control circuit The most common power-supply control scheme is current-mode pulsewidth modulation (PWM), in which variable-width pulses control the inductor’s charging current. When the load is heavy, the width of the applied pulse increases to store more energy in the inductance, as Figure 2 shows. Under no-load or light-load conditions, the width of the control pulse narrows to store less energy in the inductor. For low-current loads, the power supply operates in discontinuous mode and the main current draw is the power supply itself. The main advantage of PWM control is its ﬁxed switching frequency, which simpliﬁes the circuit design with regard to controlling EMI and optimizing efﬁciency for heavy loads. Its main drawback is the current draw under no-load and light-load conditions, because the oscillator in the regulator chip has a ﬁxed frequency (a lightly loaded UC3845, for example, draws I = 17 mA). Figure 3 shows a typical UC3845 conﬁguration in which the total curcc rent drawn is that of the main controller plus that of the voltage- and current-feedback networks. Topology of the feedback loop Voltage feedback is produced by routing current from the phototransistor (of optocoupler U45) through R135. The value of R135 must be large to minimize power, but must also be small enough to supply the minimum current needed for operation of the phototransistor. Current feedback is obtained though the voltage drop in R134. To minimize loss, we use R125 and R133 to divide the drop between this voltage and the reference (V = 5 V, pin 8), thereby allowing a 1-V equilibrium at ISENSE (pin 3). These connections improve efﬁciency by reducing the voltage drop in R135. The divider resistors must have high values to minimize power consumption, but you must also take care that the RC ﬁlter formed with C53 doesn’t affect the current signal. Power consumption in the oscillator components (R126 and C46) is unavoidable, REF because the voltage output must be maintained at all times. NEW APPROACHES TO FURTHER REDUCE CURRENT Several alternatives, based on Texas Instruments’ UCC38C41 or Maxim’s MAX5021 PWM controller and TI’s TLV431C or Maxim’s MAX8515A precision reference, can further reduce current drawn by the converter. The associated components are selected for minimum possible power consumption. Error comparator The classic TL431 can sometimes implement a precision reference. That option is not available in this case, because the resulting voltage (V = V = 2.5 V, plus drops in the U45 LED and in R124) is too close to the desired 3.6-V output. One alternative is the MAX8515A shunt regulator from Maxim. It includes a voltage reference of only 0.6 V, with 1% tolerance in the range -40ºC to +85°C. This IC is the best option for applications in which the circuit must provide lower output voltages, because it doesn’t have the limitation A-Kmin REF www.embedded.com/europe | embedded systems design europe | AUGUST – SEPTEMBER 2008 27 024-025-026-027-028-029_ESDE.indd 27 28/08/08 11:59:13 http://www.embedded.com/europe

Table of Contents Feed for the Digital Edition of Embedded Systems Design Europe - August/September 2008

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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