Low quiescent current with asynchronous buck converters at high temperatures?

December 07, 2015 // By Frank Dehmelt, Texas Instruments
Low quiescent current with asynchronous buck converters at high temperatures?
Many modern Buck-Converters are advertised with very low quiescent currents (Iq) in the order of 20 to 40uA. In reality, the “no-load-input-current” is significantly higher. At elevated temperatures, the input current may even change by orders of magnitude.

This article discusses the contributors to no-load-input current in addition to the Iq of the converter itself, and the associated temperature dependency. Furthermore, we analyze when this effect is of concern and what to do about it.

Contributors to “no-load-input-current”:

The quiescent current (Iq) of a converter is one contributor only, and also its definition and test conditions indicate that in reality, the converter will show a higher current consumption at no load. Quiescent current of a converter refers to the supply current drawn at no load, non-switching by biasing the feedback pin at reference-voltage or slightly above and potentially at room-temperature only. Consequently, non-switching indicates that no switching losses are accounted for, even though a converter will switch once in a while at no load. There will be some leakage and also a recharge of the boot-capacitors or a charge-pump is likely required, adding to the losses. A drift of the quiescent current over temperature may or may not be specified separately.

Luckily, the temperature drift of most converters is expected in the order of approximately 10% increase at higher temperatures. For test-purposes to specify the Iq of a converter, the feedback is biased to the reference voltage. Thus, the feedback-divider to sense the output voltage is eliminated. Consequently, the current flowing through it (mostly in the order of 10uA..50uA) as well as switching of the FETs is prevented and its associated losses saved. In reality, those effects do contribute to the “no-load-input-current”. The graphic illustrates the contributors that, in addition to the quiescent current, will draw current from the supply.

Figure 1: Asynchronous buck-converter and associated currents

A step-down converter with a specified quiescent current of 30 uA will – at room-temperature – most likely draw in excess of 50 uA of no load-current, accounting for feedback-divider-current and switching losses.
However, this does not explain the orders of magnitude of increase with higher temperature. This is related

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