Brushed DC Motor Drive for BTL Power Amplifier Circuit: Linear Current Drive

A resistor Rs for current detection is inserted between the motor and the output OUT1, and the current flowing through the motor is detected as a voltage and negatively fed back to the input stage amplifier. The input stage inverting amplifier performs feedback control on the output voltages of the two output stage power amplifiers according to the feedback. The output stage amplifier is a differential amplifier, the OUT1 amplifier uses the inverting input to receive the output M0 of the input stage amplifier, and the OUT2 amplifier uses the non-inverting input to receive, so OUT2 becomes the opposite phase to OUT1. Overall, it can be thought of as a single-ended input differential output (non-inverting/inverting) amplifier.

This article will describe the linear current drive of a brushed DC motor using a BTL amplifier.

The figure below is an example of a BLT amplifier circuit driving a brushed DC motor with linear current.

Brushed DC Motor Drive for BTL Power Amplifier Circuit: Linear Current Drive

A resistor Rs for current detection is inserted between the motor and the output OUT1, and the current flowing through the motor is detected as a voltage and negatively fed back to the input stage amplifier. The input stage inverting amplifier performs feedback control on the output voltages of the two output stage power amplifiers according to the feedback. The output stage amplifier is a differential amplifier, the OUT1 amplifier uses the inverting input to receive the output M0 of the input stage amplifier, and the OUT2 amplifier uses the non-inverting input to receive, so OUT2 becomes the opposite phase to OUT1. Overall, it can be thought of as a single-ended input differential output (non-inverting/inverting) amplifier.

The relationship between input voltage and output current is described below. The output current (motor drive current) is fed back to the inverting pin of the input stage inverting amplifier as the output Vso of the differential amplifier that detects and amplifies the voltage difference (amount of voltage drop) across the current detection resistor Rs. Vso is calculated by the following formula. As can be seen from the connection of Rs, Vso becomes the inverse voltage with respect to the current direction.

Vso=-Rs・Io・(R6/R5)+Vref

Assuming that the voltage applied to the input pin IN1 is VIN1, and the gain of the input stage inverting amplifier is infinite, the voltage at the inverting input pin will be Vref.

Based on the working principle of the inverting amplifier, the current sum of the inverting input pin of the inverting amplifier of the input stage is controlled to be 0A. That is, it becomes the following formula:

Brushed DC Motor Drive for BTL Power Amplifier Circuit: Linear Current Drive

After transforming this formula, it becomes

Brushed DC Motor Drive for BTL Power Amplifier Circuit: Linear Current Drive

Since Rs(R6/R5) is a fixed value, the output current Io can be set to the desired value by adjusting VIN1.

The current drive will drive the motor with the maximum voltage when the current is less than the set current, so it has the characteristics of short time to reach the set current and fast response speed of the motor. In addition, the brushed DC motor drive in this paper is introduced by taking the linear drive as an example, but the current can also be detected through the PWM output, and the current drive can be realized through the negative feedback control.

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