Linear Current Drive of Brushed DC Motors Using BTL Amplifiers

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

A resistor Rs is inserted between the motor and the output OUT1 to detect the current. 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 based on this feedback. The output stage amplifier is a differential amplifier. The OUT1 amplifier uses the inverting input terminal to receive the output M0 of the input stage amplifier, while the OUT2 amplifier uses the non-inverting input terminal to receive it, so OUT2 becomes the opposite phase to OUT1. In general, it can be regarded as a single-ended input differential output (positive phase/inverting phase) amplifier.

The following describes the relationship between input voltage and output current. 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 an inverted voltage relative 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 of the inverting input pin will be Vref.

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

After transforming the formula, it becomes

，

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

When the current drive is less than the set current, the motor will be driven at the maximum voltage, so it has the characteristics of a short time to reach the set current and a fast motor response speed. In addition, the brushed DC motor drive in this article is introduced using linear drive as an example, but the current can also be detected through PWM output and current drive can be achieved through negative feedback control.