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 a 24 bits in length while the

power fault limit register is 16 bits.

reprogrammed each time power is applied.

8.2.2.5 Calculate Returned Values

Parametric values are calculated by multiplying the returned value by the LSB value. Table 8-4 below shows the

returned values for this application example assuming the design requirements shown in Table 8-3.

Table 8-4. Calculating Returned Values

PARAMETER

Returned Value

LSB Value

Calculated Value

Shunt voltage (V)

19440d

5 µV/LSB

0.0972 V

Current (A)

19660d

5.9997 A

Bus voltage (V)

15360d

3.125 mV/LSB

48 V

Power (W)

4718604d

Current LSB x 0.2 = 61.035156 µW/LSB

288 W

Temperature (°C)

200d

125 m°C/LSB

25°C

Shunt Voltage, Current, Bus Voltage (positive only), and Temperature return values in two's complement format.

In two's complement format a negative value in binary is represented by having a 1 in the most significant bit of

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

the unsigned binary value. This value should then be converted to decimal with the negative sign applied. For

example, assume a shunt voltage reading returns 1011 0100 0001 0000. This is a negative value due to the

MSB having a value of one. Inverting the bits and adding one results in 0100 1011 1111 0000 (19440d) which

from the shunt voltage example in Table 8-4 correlates to a voltage of 97.2 mV. Since the returned value was

negative the measured shunt voltage value is -97.2 mV.

8.2.3 Application Curves

Figure 8-3 and Figure 8-4 show the ALERT pin response to a bus overvoltage fault with a conversion time of 50

μs, averaging set to 1, and the SLOWALERT bit set to 0 for bus only conversions. For these scope shots,

persistence was enabled on the ALERT channel to show the variation in the alert response for many sequential

fault events. If the magnitude of the fault is sufficient the ALERT response can be as fast as one quarter of the

ADC conversion time as shown in Figure 8-3. For fault conditions that are just exceeding the limit threshold, the

response time for the ALERT pin can vary from approximately 0.5 to 1.5 conversion cycles as shown in Figure

8-4. 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 slower than fault events starting from values near the set fault threshold. Since the timing

of the alert can be difficult to predict, applications where the alert timing is critical should assume a alert

response equal to 1.5 times the ADC conversion time for bus voltage or shunt voltage only conversions.

www.ti.com

INA239

SLYS027 – JANUARY 2021

Copyright © 2021 Texas Instruments Incorporated

Submit Document Feedback

33

Product Folder Links: INA239