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An overcurrent threshold is set by programming the Current Over-Limit Threshold register (COL). Divide the

overcurrent limit value by the current LSB size to calculate the value needed to program the register.

In this example, the desired overcurrent limit threshold is 42 A. The Current LSB size is 2.4 mA/LSB, therefore

the value that should be programmed into Current Over-Limit (COL) register the value is 42 A / 2.4 mA/LSB =

17500d or 445Ch.

An overvoltage fault threshold on the bus voltage is set by programming the bus overvoltage limit register

(BOVL). In this example, the desired overvoltage threshold is 54 V. Divide the target threshold voltage by the

correct LSB value to calculate the value needed to program the register. For this example, the target value for

the BOVL register is 54 V / 3.125 mV = 17280d (4380h).

When setting the power over-limit value, the LSB size used to calculate the value needed in the limit registers

will be 256 times greater than the power LSB. This is because the power register is 24 bits in length while the

power fault limit register is 16 bits.

Values stored in the alert limit registers are set to the default values after VS power cycle events and must be

reprogrammed each time power is applied.

7.2.2.3 Calculate Returned Values

Multiply the returned value by the LSB value to calculate the parametric values. Table 7-4 below shows the

returned values for this application example assuming the design requirements shown in Table 7-3.

Table 7-4. Calculating Returned Values

PARAMETER

RETURNED VALUE

LSB VALUE

CALCULATED VALUE

Current (A)

15000d, 3A98h

2.4 mA/LSB

36 A

Bus voltage (V)

15360d, 3C00h

3.125 mV/LSB

48 V

Power (W)

3600000d, 36 EE80h

480 µW/LSB

1.728 kW

Energy (J)

202500000d, C11E7A0h

7.68 mJ/LSB

1.5552 MJ

Charge (C)

13500000d, CDFE60h

150 µC/LSB

32400 C

Temperature (°C)

680d, 2A8h

125 m°C/LSB

85°C

Current, Bus Voltage (positive only), Charge, and Temperature return values in 2's complement format. In a 2's

complement format, a 1 in the most significant bit of the returned value represents a negative value in binary.

These values can be converted to decimal by first inverting all the bits and adding 1 to obtain the unsigned

binary value. This value must then be converted to decimal with the negative sign applied.

7.2.3 Application Curves

Figure 7-3 and Figure 7-4 show the ALERT pin response to an overcurrent fault with a conversion time of 50 μs

for the temperature, shunt voltage, and bus voltage measurements with averaging set to 1. This configuration

results in a total conversion time of 150 μs for all three measurements. For these scope shots, persistence was

enabled on the ALERT channel to show the variation in the alert response for many sequential fault events.

The alert response time can change depending on the value of the current before fault occurs as well as the

how much the fault condition exceeds the programmed fault threshold. Figure 7-3 shows the response time

for an overcurrent fault when the fault condition greatly exceeds the programmed threshold, while Figure 7-4

shows the overcurrent response time when the fault slightly exceeds the programmed threshold. Variation in the

alert response exists because the external fault event is not synchronized to the internal ADC conversion start.

Also the ADC is constantly sampling to get a result, so the response time for fault events starting from zero

will be slower than fault events starting from values near the set fault threshold. In applications where the alert

timing is critical for overcurrent events, the worst-case alert response is equal to 2 × tconv_current + tconv_temp +

tconv_voltage + 25 μs. An additional 25 μs is added to allow for background math calculations. This equation does

not account for the 1% oscillator tolerance and is only valid for cases where the overcurrent signal is greater

than the conversion threshold and noise. The measurement noise is a function of the conversion time. See

Section 6.3.5.2 and Section 7.1.2 for additional information.

INA780A, INA780B

SBOSAB6A – JULY 2023 – REVISED DECEMBER 2023

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