Low Voltage Low Noise HTC863XA/HTC72XA Rail-to-Rail Series Operational Amplifiers Introduction

The HTC863XA and HTC72XA are low-noise, low-voltage, low-power operational amplifiers. These devices can be used in a wide range of applications. Their input common-mode voltage range extends to ground, with maximum input offset voltages of 4.2 mV and 3.5 mV, respectively. Under heavy loads, they provide rail-to-rail output swing. The HTC863XA operates from a single supply voltage ranging from +2.3 V to +5.5 V, while the HTC72XA operates from +2.5 V to +5.5 V. Both operate over a temperature range of -40°C to +125°C.

The HTC8631A and HTC721A are available in 5-pin SC70 and SOT-23 packages. The HTC8632A and HTC722A come in 8-pin MSOP, DFN2×2, TSSOP, and SO packages. The HTC8634A and HTC724A are offered in 14-pin TSSOP and SO packages.

• Photodiode amplifiers

• Sensor interfaces

• Audio outputs

• Active filters

• A/D converters

• Handheld devices

• Battery-powered equipment

In applications such as inverters and servo drives that require Field-Oriented Control (FOC), operational amplifiers are commonly used to amplify phase currents. The analysis is as follows:

Sampling motor phase current is essential for FOC. When designing motor control circuits, it is critical to enhance the anti-interference capability of current acquisition to accurately measure the current flowing through the motor windings. Current sensing is a vital part of motor drive control development, and precise current sampling is a prerequisite for reliable motor operation.

Current sensing serves two main purposes: First, to ensure rapid motor startup performance by monitoring the main current signal in the motor circuit, enabling the controller to generate accurate PWM signals for closed-loop current control. Second, to promptly detect faults such as short circuits or overcurrent during actual operation and relay this information to the controller, which then issues a command to shut down the switches immediately to protect the hardware. A typical motor drive system should include DC bus current sensing, motor phase current sensing circuits, and may also incorporate dedicated current sensing circuits to verify the proper operation of specific modules. Therefore, designing an accurate and effective current sensing circuit is key to successful motor drive control system design.

In motor control, MOSFETs are typically switched on and off using PWM signals, with the duty cycle adjusted to regulate motor current. A higher duty cycle results in greater motor current and faster rotation; conversely, a lower duty cycle slows the motor down. To enable control of the motor even at low speeds, the MCU’s ADC must be capable of sampling the motor current at very low duty cycles—typically as low as 5%. For example, with a 20 kHz PWM signal (period = 50 µs), a 5% duty cycle corresponds to 2.5 µs. However, op-amps require a finite amount of time to respond to sudden voltage changes, characterized by their slew rate (SR) and settling time (TSET). Taking the HTC8632A as an example, its SR is 4 V/µs and TSET is 1 µs. Thus, an amplified voltage signal rising from 0 V to 3.3 V requires 3.3 / 4 = 0.825 µs, followed by an additional 1 µs to settle before the ADC can accurately sample it. This leaves only 2.5 – 0.825 – 1 = 0.675 µs for ADC sampling (TSMP).

To ensure the ADC can accurately and timely capture valid signals, a longer TSMP is desirable. This implies that the op-amp should have a higher slew rate (larger SR value) and a shorter settling time (smaller TSET value).

Multiple package options available:

HTC863XA and HTC72XA Parameter Comparison Table

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