Embedded Systems Design - July 2008 - (Page 30) feature This output-divider circuit provides an error-comparator signal for the isolated switchmode power supply. T2 D58 +3V6 not producing a regulated output voltage. Finally, you may need to add galvanic isolation to ensure efficient protection for devices designed to operate in hostile environments. DESIGN GOALS AND ISSUES In the design of a power supply for portable or wireless devices, three key technical requirements should be taken into account: very low no-load power consumption, isolation, efficiency, and size. Issues that arise in meeting these requirements involve the correct isolation configuration, the control method, and the topology of the feedback loop Isolation Isolation between input and output is achieved with a transformer, and for inverting or flyback configurations, the energy is stored in the transformer’s inductance. The problem, then, is how to provide feedback from the secondary to the primary. The majority of systems use an extra auxiliary winding or optocoupler for that purpose. An auxiliary winding, however, increases the complexity while not providing enough output-voltage precision for low outputs and variable loads. When the system is in regulation, an optocoupler needs constant current through the primary-side LED. To optimize the system, this current has been minimized as much as possible, as Figure 1 shows. The minimum limit is set by a reduction of the optocoupler’s current transfer ratio (CTR) at low current (63% at 10 mA and 22% at 1 mA), and a reduction of speed (2 µs at 20 mA and 6.6 µs at 5 mA). We must add yet another limitation, which is the minimum current (Ikmin = 100 µA) that must flow from the error comparator through the precision shunt regulator TLV431. For the resistive divider connected to the shunt regulator’s output (R131 and R137), high-value resistors have been selected to minimize the current draw. You must take care to compen- R124 U45 R127 R130 R131 C44 U42 TLV431A R137 0 Figure 1 + C47 Pulse-width modulation (PWM) control produces a control voltage (middle trace) and inductor current (bottom trace) in response to changes in load current (top trace). IO Duty cycle Vctrl t IL T T T T T T T T T T T T T T t PWM => T=constant Figure 2 IO => t ON 30 JULY 2008 | embedded systems design | www.embedded.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 Embedded Systems Design - July 2008 - Embedded Systems Design - July 2008 (Page Cover1) Embedded Systems Design - July 2008 - Embedded Systems Design - July 2008 (Page Cover2) Embedded Systems Design - July 2008 - Embedded Systems Design - July 2008 (Page 1) Embedded Systems Design - July 2008 - Embedded Systems Design - July 2008 (Page 2) Embedded Systems Design - July 2008 - Contents (Page 3) Embedded Systems Design - July 2008 - Contents (Page 4) Embedded Systems Design - July 2008 - Contents (Page 5) Embedded Systems Design - July 2008 - Contents (Page 6) Embedded Systems Design - July 2008 - #Include (Page 7) Embedded Systems Design - July 2008 - #Include (Page 8) Embedded Systems Design - July 2008 - Parity Bit (Page 9) Embedded Systems Design - July 2008 - Programming Pointers (Page 10) Embedded Systems Design - July 2008 - Programming Pointers (Page 11) Embedded Systems Design - July 2008 - Programming Pointers (Page 12) Embedded Systems Design - July 2008 - Programming Pointers (Page 13) Embedded Systems Design - July 2008 - Programming Pointers (Page 14) Embedded Systems Design - July 2008 - Programming Pointers (Page 15) Embedded Systems Design - July 2008 - Programming Pointers (Page 16) Embedded Systems Design - July 2008 - Programming Pointers (Page 17) Embedded Systems Design - July 2008 - Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design (Page 18) Embedded Systems Design - July 2008 - Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design (Page 19) Embedded Systems Design - July 2008 - Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design (Page 20) Embedded Systems Design - July 2008 - Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design (Page 21) Embedded Systems Design - July 2008 - Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design (Page 22) Embedded Systems Design - July 2008 - Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design (Page 23) Embedded Systems Design - July 2008 - Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design (Page 24) Embedded Systems Design - July 2008 - Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design (Page 25) Embedded Systems Design - July 2008 - Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design (Page 26) Embedded Systems Design - July 2008 - Building a Power Supply for Discontinuous Transmission Wireless Networks (Page 27) Embedded Systems Design - July 2008 - Building a Power Supply for Discontinuous Transmission Wireless Networks (Page 28) Embedded Systems Design - July 2008 - Building a Power Supply for Discontinuous Transmission Wireless Networks (Page 29) Embedded Systems Design - July 2008 - Building a Power Supply for Discontinuous Transmission Wireless Networks (Page 30) Embedded Systems Design - July 2008 - Building a Power Supply for Discontinuous Transmission Wireless Networks (Page 31) Embedded Systems Design - July 2008 - Building a Power Supply for Discontinuous Transmission Wireless Networks (Page 32) Embedded Systems Design - July 2008 - Building a Power Supply for Discontinuous Transmission Wireless Networks (Page 33) Embedded Systems Design - July 2008 - Building a Power Supply for Discontinuous Transmission Wireless Networks (Page 34) Embedded Systems Design - July 2008 - An Exception Primer (Page 35) Embedded Systems Design - July 2008 - An Exception Primer (Page 36) Embedded Systems Design - July 2008 - An Exception Primer (Page 37) Embedded Systems Design - July 2008 - An Exception Primer (Page 38) Embedded Systems Design - July 2008 - An Exception Primer (Page 39) Embedded Systems Design - July 2008 - An Exception Primer (Page 40) Embedded Systems Design - July 2008 - An Exception Primer (Page 41) Embedded Systems Design - July 2008 - An Exception Primer (Page 42) Embedded Systems Design - July 2008 - An Exception Primer (Page 43) Embedded Systems Design - July 2008 - Advertising Index (Page 44) Embedded Systems Design - July 2008 - Break Points (Page 45) Embedded Systems Design - July 2008 - Break Points (Page 46) Embedded Systems Design - July 2008 - Marketplace (Page 47) Embedded Systems Design - July 2008 - Marketplace (Page 48) Embedded Systems Design - July 2008 - Marketplace (Page Cover3) Embedded Systems Design - July 2008 - Marketplace (Page Cover4)
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