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Reduced Current Sense Offset Errors in a Motor Power Driver

IP.com Disclosure Number: IPCOM000036761D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 2 page(s) / 41K

IBM

Related People

Valent, JA: AUTHOR

Abstract

This article describes a circuit which enables accurate current sensing in a motor power driver that is configured in an H-bridge arrangement operating between positive and negative power supplies.

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Reduced Current Sense Offset Errors in a Motor Power Driver

This article describes a circuit which enables accurate current sensing in a motor power driver that is configured in an H-bridge arrangement operating between positive and negative power supplies.

The figure illustrates a typical H-bridge topology with the disclosed circuit contained in the dashed box and labeled "Current Sense Circuit". The configuration shown is for sensing current magnitude only. Magnitude and polarity can be obtained by using two current sense resistors and two of the current sense circuits feeding a differential amplifier.

When dual power supplies are used, current sensing is accomplished by measuring the voltage across a small resistor, RS, that is inserted in series with the load; one end of the resistor is connected to the negative power supply. The current sense circuit operates at the negative supply and performs a level shift so that the rest of the power driver can operate around ground. Amplifier AI must be of the type that can operate at the negative rail, such as the LM324 (National Semiconductor Corp.). The current that flows through RS is scaled via R1 and mirrored into the collector of transistor Q5. The level shift is accomplished by the collector of Q5 and resistor R2 reference, ground for the case shown in the figure. The voltage VS at summer SI due to current sensing is: VS = -(R2/R1) x IM x RS + VOS x (R2/R1)

VOS is the inherent input offset voltage of amplifier A...