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

April 16, 2014 // By David Rice
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.

The power dissipated can increase temperatures beyond the recommended operating limits and can be a major concern for systems with large channel density or with higher ambient temperatures.

For example, a DAC needs to supply up to 20 mA to a user-defined load in the 100Ω to 1 kΩ range. In this case the minimum supply voltage must be 20V. The maximum power supplied by the DAC is V × I = 20V × 20 mA = 0.4W. If a 1-kΩ load is used, all of the power is consumed by the load, resulting in no lost power. A 100-Ω load consumes only 0.04W, so 0.36W is wasted or dissipated on chip. In some cases, a 0-Ω load is a valid condition, resulting in all power being dissipated on chip.

In a 64-pin LFCSP package, the maximum ambient temperature cannot exceed 125°C. With four channels each dissipating 0.4W, the total power dissipated is 1.6W. The thermal impedance of a 64-LFCSP package is 28ºC/W. In the previous example, the temperature rise is PD × θJA = 1.6W × 28°C/W = 44.8°C. Therefore the maximum safe ambient temperature is only 80.2°C. Heat sinks can be added to overcome this problem, but this may not be viable due to the required space and cost.

Dynamic power control (DPC) directly addresses this issue. A dc-to-dc converter boosts a 5-V supply to create a 7.5-V to 29.5-V supply. This boosted supply powers the DAC current output driver, which delivers the required power to the load. With a 0-Ω load, the output of the DC/DC converter is 7.5V, its lowest value. The maximum power dissipated in the DAC is only 7.5V × 20 mA = 0.15W, saving 0.25W compared to the original solution.

With DPC, the maximum power dissipated by four channels (each short-circuited to ground) is 0.6W. The temperature rise is PD × θJA = 0.6W × 28°C/W = 16.8°C. Therefore the maximum safe operating temperature increases to 108.2°C. DPC provides the most benefit in systems having a wide undefined load range, high channel density, and high temperatures that leave little room for large power losses.


next; a 4-channel, 16-bit DAC with DPC

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