Design Con 2015

Some thoughts on DC/DC converters, part two

Bob Dobkin and Jim Williams, Linear Technology -March 27, 2013

Some thoughts on DC/DC converters, part two


The following is Chapter 4, part two "Some thoughts on DC/DC converters" by the late Jim Williams and Brian Huffman from Linear Technology's Volume II book entitled, "Analog Circuit Design-- Immersion in the Black Art of analog design" by Bob Dobkin and the late-Jim Williams published by Elsevier/Newnes.

Part one and was published previously. Part three, Part four and Part five are published online as well.

Micropower quiescent current converters

Many battery-powered applications require very wide ranges of power supply output current. Normal conditions require currents in the ampere range, while standby or “sleep” modes draw only microamperes. A typical laptop computer may draw 1 to 2 amperes running while needing only a few hundred microamps for memory when turned off. In theory, any DC/DC converter designed for loop stability under no-load conditions will work. In practice, a converter’s relatively large quiescent current may cause unacceptable battery drain during low output current intervals.

Figure 4.11 shows a typical flyback based converter. In this case the 6V battery is converted to a 12V output by the inductive flyback voltage produced each time the LT1070’s VSW pin is internally switched to ground (for commentary on inductor selection in flyback converters see Appendix D, “Inductor Selection for Flyback Converters”). An internal 40kHz clock produces a flyback event every 25µs. The energy in this event is controlled by the IC’s internal error amplifie r, which acts to force the feedback (FB) pin to a 1.23V reference. The error amplifiers high impedance output (the VC pin) uses an RC damper for stable loop compensation.

 

Figure 4.11 • 6V to 12V, 2 Amp Converter with 9mA

This circuit works well but pulls 9mA of quiescent current. If battery capacity is limited by size or weight this may be too high. How can this figure be reduced while retaining high current performance?

A solution is suggested by considering an auxiliary VC pin function. If the VC pin is pulled within 150mV of ground the IC shuts down, pulling only 50 microamperes. Figure 4.12’s special loop exploits this feature, reducing quiescent current to only 150 microamperes. The technique shown is particularly significant, with broad implication in battery powered systems. It is easily applied to a wide variety of DC/DC converters, meeting an acknowledged need across a wide spectrum of applications.


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