1 Introduction

Isolated amplifiers can measure voltage or current with relatively high accuracy while keeping the measurements isolated from the low-side. This is useful in applications where the high-side voltage requires isolation for safety-related concerns, or when the high-side can experience sudden transients which can damage a controller on the low-side. Common applications include measuring a high-voltage motor bus or measuring motor phase current.

However, isolated amplifiers require the high-side power supply to be isolated from the low-side power supply, which can lead to increased size and complexity. One alternative is a transformer-isolated power supply, which produces the high-side rail from the low-side while keeping the high-side isolated from the low-side. However, transformers can be large and costly. A bootstrap charge-pump power supply is a cost-effective alternative. The power is supplied from a pulse-width modulation (PWM) signal, and only requires a capacitor, a diode, and a current-limiting resistor. In some cases, a linear dropout regulator (LDO) can be required as well.

Figure 1-1 Bootstrap Power Supply