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OPA607

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SBOS981 – OCTOBER 2019

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dissipated across a surface-mount device (SMD) resistor on a board having a good thermal layout is

approximately 1 W.

V. This value can be found in the Electrical Characteristics table of the ADS7042 data sheet. However, for this

design, the maximum allowable voltage at the input of the ADC under normal operating condition is assumed to

be 3 V. Thus, the full-scale reading (FSR) for this design can be considered as 3 V. This value enables the

design to have a 300-mV headroom in case the current exceeds the maximum rated value of 12 A. In other

words, for faults and other unexpected conditions, the ADC is capable in measuring currents up to 15 A. The

circuit. Because the considered FSR is 3 V, the output of the OPA607 must swing from the nominal value of 1.24

Equation 4 defines the signal gain required by the op amp, which results in a gain of approximately 20 V/V.

Gain = (3 V – 1.24 V) / 84 mV

(4)

nominal 1.24 V proportionally. If the gain value is frozen at 20 V/V, then Equation 5 defines the maximum current

design requirement of current measuring capability of 8 A from GND to load.

(5)

The next step is to arrive at the right GBW of an op amp that can drive a SAR ADC, while still being able to gain

the signal by 20 V/V. As discussed further in this section, the calculations point towards an op amp with GBW in

the vicinity of 50 MHz.

A 20-kHz switching system such as a motor driver card has a minimum possible duty cycle of approximately 5%.

Thus when the control card is running at its lowest duty cycle, the width of the current pulse (as shown in

Figure 8) shrinks to approximately 2.5 µs. The current pulses are very narrow and require the op amp output to

settle to the final value within this short span of time. Figure 8 is an exaggerated figure showing the typical

current waveform in a low-side current-sensing signal chain of a sinusoidal-driven brushless DC electric motor

(BLDC). An op amp must be able to settle to its 0.1% accuracy within this time period to ensure the current value

very desirable because the same amplification circuit can be fed to the comparator for short-circuit protection.

This feature brings as additional value of using a high-bandwidth op amp in a current-sensing circuit. Because of

the high bandwidth of the OPA607, as with any op amp, the rise time (which is the prime parameter deciding the

response time of the short circuit protection circuitry) becomes very small.

Equation 6 describes how the rise time and the bandwidth of a op amp are related to each other.

(6)