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9.2.4 Low-Cost, Low Side, High-Speed Current Sensing

3.3V

1NŸ

VREF =1.24V

ISH

LOAD

OPA607

688 Ÿ

240 pF

GND

3.3V

20 NŸ

RSH

ADS7042

VOUT

VADC

Figure 9-12. Low Side Current Sensing

9.2.4.1 Design Requirements

The objective is to design a high-speed, high-gain bidirectional current-sensing circuit for power systems and

motor drive systems. Section 9.2.4.2 lists the design requirements of this application.

Table 9-3. Design Parameters

PARAMETER

DESIGN REQUIREMENT

Amplifier and ADC supply

3.3 V

Peak current to be measured from load to ground

20 A

Peak current to be measured from ground to load

12 A

Required Accuracy of current measurement

0.1%

Signal-Setting time at ADC input

< 1 µs

Current sensing direction

Bidirectional

9.2.4.2 Detailed Design Procedure

The aim of this application section is to measure bidirectional current with relatively high accuracy in a low-side-

sensing-based, high-frequency switching system.

As shown in Figure 9-12, a single op amp of high bandwidth is capable of sensing current in a high gain

configuration as well as have the required effective bandwidth to drive the consecutive SAR ADC input. The SAR

ADC can be a standalone ADC or integrated inside a Micro-controller.

VOUT = (20 kΩ / 1 kΩ × VDIFF) + VREF, where VDIFF = ISH X RSH

(2)

The reference voltage is 1.24 V. When the ISH flowing across RSH equals zero, the VOUT of the difference

amplifier sits ideal at 1.24 V.

When the current (ISH) flows from LOAD to GND, the output of the OPAx607 increase above 1.24 V with a value

equal to 20 × VSH and when the current flows from GND to LOAD (in the opposite direction) the output of the

OPAx607 decrease below 1.24 V with a value proportional to 20 × VSH.

One of the main challenges in a high speed current sensing design is to choose an op amp of with sufficient

GBW that can drive a SAR ADC, while still being able to gain the signal by the required amount. The 0.1% and

0.01% settling of OPAx607 can found in Section 7.5. Another key care about is to ensure the op amp output

rises in less than 1 µs so as to feed the output to a comparator for short-circuit protection. This comparator

based short circuit protection loop is extremely fast and enables to turn off the switching devices very quickly.

This requirement makes a low cost high speed part like the OPAx607 very desirable in a current-sensing circuit.

Equation of the rise time as a function of bandwidth is shown below.

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OPA607, OPA2607

SBOS981F – OCTOBER 2019 – REVISED SEPTEMBER 2020

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