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Texas Instruments Low Quiescent Current With Asynchronous Buck-Converters at High Temperatures Application notes
Application Report
SLVA810 – August 2016
Low Quiescent Current with Asynchronous Buck
Converters at High Temperatures
ABSTRACT
This application report provides a summary of the Low Quiescent Current with Asynchronous Buck
Converters at High Temperatures for TPS65320/1-Q1 devices.
1
Introduction
Many modern Buck Converters are advertised with very low quiescent currents (Iq) in the order of
between 10 and 40 µA. In reality, the no-load input current is significantly higher. At elevated
temperatures, the input current may even change by orders of magnitude.
This application report discusses the contributors to no-load input current in addition to the Iq of the
converter itself and associated temperature dependency. Furthermore, this analyzes when this effect is of
concern and what to do about it.
1.1
Contributors to No-Load Input Current
The Iq of a converter is one contributor only and its definition and test conditions also 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 occasionally at no load.
There will be some leakage and also a recharge of the boot-capacitors or a charge-pump is likely
required, which adds 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 an
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, this eliminates the feedback-divider to sense the output
voltage. Consequently, the current flowing through it (mostly in the order of 10 µA..50 µA) as well as
switching of the FETs is prevented and associated losses saved. In reality, those effects do contribute to
the no-load input current. Figure 1 illustrates the contributors that, in addition to the quiescent current,
draw current from the supply.
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Low Quiescent Current with Asynchronous Buck Converters at High
Temperatures
Copyright © 2016, Texas Instruments Incorporated
1
Introduction
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Supply /
Input
Current
Quiescent
Current
Control
Gate-Drive
Current
Gate Drive
Vout
Reverse
Blocking
Current
Load
Current
Feedback
Current
Figure 1. Asynchronous Buck Converter and Associated Currents
A step-down converter with a specified quiescent current of 30 µA will – at room-temperature – most likely
draw in excess of 50 µA of no load-current, which accounts for feedback-divider-current and switching
losses.
However, this does not explain the orders of magnitude of increase with higher temperature as observed
with asynchronous converters: here, the catch-diode plays a significant role in this case: the majority of
diodes show an increased reverse current, mostly about 100 times higher at higher temperatures (85°C or
higher) compared to room temperature.
For low-voltage and low-current diodes (for example: 10 V, 500 mA), one can find diodes with as low as 1
µA-rated reverse current at room temperature, which consequently rises to about 100 µA at higher
temperatures. Unfortunately, for low loads, blocking will be the diode status for the vast majority of the
time. For higher-voltage and higher-current diodes (40 V, 4 A), the few microamperes or few dozen
microamperes at room temperature will account for several milliamperes at elevated temperatures.
Another factor to consider is the increase in reverse current with increased reverse voltage. Here a rise of
about one decade has to be expected from low voltages to maximum-rated voltages. However, clearly
defined diode data sheets usually specify the maximum-reverse current at maximum-rated blocking
voltage (and at discrete temperatures). Consequently, any asynchronous converter will exhibit a
significantly higher no-load current at higher temperatures, heavily dominated by the catch-diode and its
increase of reverse current with temperature.
2
Low Quiescent Current with Asynchronous Buck Converters at High
Temperatures
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SLVA810 – August 2016
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Introduction
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1.2
When This is a Concern
Obviously, if the respective system is not exposed to relatively high temperatures, this is of no concern.
Similarly, if the system is only operating in normal mode, hence normal switching operation as opposed to
power-save or no-load mode at higher temperatures, this may be negligible. An example could be an
automotive application, where a unit such as the front-view-camera, is either off when hot, like for a car
parked in the sun, or in normal operation when the car is running and the alternator is working, and
therefore a small loss in efficiency can be tolerated. Similarly in industrial applications, a system is hardly
ever in standby mode, but either off or in normal mode. For consumer systems that are battery driven, it
could be applicable depending on the expected temperatures. To what extent the effect is seen also
depends on the blocking voltage, hence the battery voltage in most cases and load-current that drives the
diode selection.
1.3
What to Do if This Effect is of Concern
Keep cool! Admittedly, this is easier said than done. A more appropriate solution is to choose a
synchronous converter, which eliminates the diode and replaces it with an active FET. A slight increase in
quiescent current still needs to be expected with temperature, but it is approximately 10% with a
synchronous converter, rather than orders of magnitude with the asynchronous converter and catch diode.
1.4
Conclusion
The quiescent current is only one contributor to the no-load-current consumption. For an asynchronous
converter, the catch-diode’s reverse current is a significant contributor, in particular at high temperatures.
In some applications, this may be irrelevant or tolerable. In case a low-quiescent current at elevated
temperatures is of concern, a synchronous converter is likely the better choice.
2
References
•
•
•
IQ: What it is, what it isn’t, and how to use it
Efficiency of synchronous versus nonsynchronous buck converters
Low Quiescent Current with Asynchronous Buck Converters at High Temperatures?
Revision History
DATE
REVISION
NOTES
August 2016
*
Initial release
SLVA810 – August 2016
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Revision History
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3
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