Enhanced OTA Models Improve Design Of Feedback Compensation Networks
by Alain Laprade, ON Semiconductor, East Greenwich, R.I, How2Power Today, Jul 15 2014
Focus:
An operational transconductance amplifier (OTA) generates a current-source output that is
proportional to a differential input voltage. To achieve ESD robustness in OTAs, a current-
limiting series protection resistor and voltage clamp are installed between the die’s OTA
output and the package pinout. Device manufacturers consider the influence of this ESD
protection resistor as negligible and the parameter is not described in datasheets. However,
when designing power supply circuits, neglecting to consider the influence of the ESD
protection resistor on OTA output impedance may introduce gain and phase errors in the feedback
loop compensation. For example, the presence of the ESD resistor affects the performance of
boost converters operating from a low voltage input. This article presents derivations of the
transfer functions for power supply OTA compensation that include the effects of this resistor.
The three common forms of compensation—types 1, 2, and 3—are considered here. In addition to
deriving the corrected transfer functions for each type of compensation, ESD correction factors
are derived, which allows the designer to obtain the feedback control transfer functions
experimentally. Parameters from the NCV8871 boost controller IC OTA will be used for supporting
design examples.
What you’ll learn:
- How to account for the effects of the ESD protection resistor found in OTAs on power supply compensation
- How to accurately model an operational transconductance amplifier (OTA) including the effects of the ESD protection resistor
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