INA233 datasheet(17/48 Pages) TI1

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Programming (continued)

7.5.4 Reading Telemetry Data and Warning Thresholds

Conversion from direct format to real-world dimensions of current, voltage, and power is accomplished by

determining the appropriate coefficients as described in section 7.2.1 of the PMBus Power System Management

Protocol Specification 1.3 Part II. According to this specification, the host system converts the received values

using Equation 3 into a reading of volts, amperes, watts, or other such units.

where

•

X = the calculated real-world value (volts, amps, watts, and so forth)

•

m = the slope coefficient

•

Y = a 2-byte, two's complement integer received from the device

•

b = the offset, which is a 2-byte, two's complement integer

•

R = the exponent, which is a 1-byte, two's complement integer

•

R is only necessary in systems where m is required to be an integer (for example, where m can be stored in a

(3)

The values for m and R (listed in Table 1) must be calculated for current and power measurements based off the

selected value of the Current_LSB. For example, assume a Current_LSB of 0.75 mA/bit is selected for a given

application. The value for m is calculated by inverting the LSB value (for this case, m = 1 / 0.00075 = 1333.333).

Moving the decimal point so the value of m is maximized and remains within the required range of –32768 to

32767 is preferable because this value of m is relatively small and contains decimal information. Moving the

decimal point one place to the right results in a final m value of 13333 with an R value of –1 resulting from the

shift in decimal location. Moving the decimal point to maximize the value of m is critical to minimize rounding

errors. The m coefficient for power can be calculated by applying 1 / (25 × Current_LSB). For this example, the

value for the m power coefficient is calculated to be 53.333. Again (to maximize accuracy), the decimal location

is shifted by 2 to the right to give a final m value of 5333 with an R coefficient of –2. Care must be taken to adjust

the exponent coefficient, R, such that the value of m remains within the range of –32768 to 32767. However,

rounding errors resulting from the limitations on the value of m can be mitigated by carefully selecting a slightly

higher current LSB size. For example, if a Current_LSB of 1 mA/bit is selected instead of 0.75 mA/bit, the

calculated value for m is 1 / 0.001 or 1000; because this value is a whole number there is no rounding errors and

the value for R is 0. Positive values for R signify the number of times the decimal point is shifted to the left,

whereas negative values for R signify the number of decimal point shifts to the right.

Table 1. Telemetry and Warning Conversion Coefficients (RS in mΩ)

COMMANDS

FORMAT

NUMBER OF

DATA BYTES

UNIT

READ_VIN

VIN_OV_WARN_LIMIT

VIN_UV_WARN_LIMIT

DIRECT

READ_IIN, READ_IOUT

MFR_IIN_OC_WARN_LIMIT

DIRECT

Calculated from

Current_LSB

Calculated

READ_PIN, READ_EIN

MFR_PIN_OP_WARN_LIMIT

DIRECT

Calculated from

Current_LSB

Calculated

MFR_READ_VSHUNT

DIRECT

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