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Power management

Analog Tip: Dynamic power control minimises power loss, maximises temperature range

April 16, 2014 | David Rice | 222907892
Analog Tip: Dynamic power control minimises power loss, maximises temperature range David Rice of Analog Devices explains that DACs in industrial systems may be expected to drive a wide range of loads. DACs powered by a fixed supply can dissipate significant power on chip, particularly if the load is small or a short circuit to ground.

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The AD5755 four-channel 16-bit digital-to-analogue converter provides voltage and current outputs for programmable logic controllers (PLC), distributed control systems (DCS), and other industrial process-control applications. Dynamic power control regulates the voltage on the output driver, minimising power dissipation with low-value load resistors—and easing thermal management. Each channel can be configured to provide:

- Voltage output, with 0V to 5V, 0V to 10V, ±5V, or ±10V full-scale range and ±0.04% total unadjusted error (TUE);

- Current output, with 0 mA to 20 mA, 4 mA to 20 mA, or 0 mA to 24 mA full-scale range and ±0.05% TUE.

Offset and gain can be individually programmed for each channel. The devices can be used with the on-chip 5 V, ±5-ppm/°C reference or an external reference. Available in a 9 × 9 × 0.85-mm 64-lead LFCSP package, it is specified from –40°C to +105°C and priced at $13.65 (1000).

Figure 1 shows its current output circuitry, DC/DC converter, and power controller. When the current output is enabled, VDS of the output FET is sensed. This voltage controls the MOSFET in the power control block to regulate VBOOST, which in turn controls VDS as determined by the output current requirement. With the MOSFET switched on, the inductor charges to a value determined by the difference in the actual value of VDS and the required value. When switched off, the inductor discharges into the capacitor and VBOOST pin. This process repeats on each clock cycle. There is one dc-to-dc converter per channel.

David Rice is an Applications Engineer at Analog Devices, Limerick, Ireland. He holds a Masters in Engineering in Embedded Systems from Cork Institute of Technology, Ireland.

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