ADM1275-2ARQZ-R7 datasheet(33/48 Pages) AD

33 / 48 page

Data Sheet

ADM1275

Rev. D | Page 33 of 48

Table 9. PMBus Conversion to Real-World Coefficients

Coefficient

Current (A)

Voltage (V)

0 V to 6 V Range

0 V to 20 V Range

807 × RSENSE

6720

19,199

20,475

−1

−2

Example 1

IOUT_OC_WARN_LIMIT requires a current-limit value

expressed in direct format.

If the required current limit is 10 A, and the sense resistor is

2 mΩ, then the first step is to determine the voltage coefficient.

This is simply m = 807 × 2, giving 1614.

Using Equation 1 and expressing X, in units of Amps

Y = ((1614 × 10) + 20,475) × 10−1

Y = 3661.5 = 3662 (rounded up to integer form)

Writing a value of 3662 with the IOUT_OC_WARN_LIMIT

command sets an overcurrent warning at 10 A.

Example 2

The READ_IOUT command returns a direct format value of

3339 representing the current flowing through a sense resistor

of 1 mΩ.

To convert this value to the current flowing, use Equation 2, with

m = 807 × 1.

X = 1/807 × (3339 × 101 − 20,475)

X = 16.00 A

This means that when READ_IOUT returns a value of 3339,

16.00 A is flowing in the sense resistor.

VOLTAGE AND CURRENT CONVERSION USING

LSB VALUES

The direct format voltage and current values returned by the

READ_VIN, READ_VOUT, READ_IOUT commands, and

the corresponding peak versions, are actually the data output

directly by the ADM1275 ADC. As the voltages and currents

are really a 12-bit ADC output code, they can also be converted

to real-world values with knowledge of the size of the LSB on

the ADC.

The m, b, R coefficients defined for the PMBus conversion are

required to be whole integers by the standard, and have therefore

been rounded-off slightly. Using this alternative method, with

the exact LSB values, can provide slightly more accurate

numerical conversions.

To convert an ADC code to current in amperes, the following

formulas can be used:

VSENSE = LSB25mV × (IADC − 2048)

IOUT = VSENSE/(RSENSE × 0.001)

where:

VSENSE = (VSENSE+) − (VSENSE−).

LSB25mV = 12.4 µV.

IADC is the 12-bit ADC code.

IOUT is the measured current value in amperes.

RSENSE is the value of the sense resistor in milliohms.

To convert an ADC code to a voltage, the following formula can

be used:

VM = LSBxV × (VADC + 0.5)

where:

VM is the measured value in volts.

VADC is the 12-bit ADC code.

LSBxV values are based on the voltage range (see Table 10).

Table 10. Voltage Ranges and LSB Values

Voltage Range, LSBxV

LSB Magnitude

0 V to 6 V

1.488 mV

0 V to 20 V

5.208 mV

To convert a current in amperes to a 12-bit value, the following

formula can be used (round the result to the nearest integer):

VSENSE = IA × RSENSE × 0.001

ICODE = 2048 + (VSENSE/LSB25mV)

where:

VSENSE = (VSENSE+) − (VSENSE−).

IA is the current value in amperes.

RSENSE is the value of the sense resistor in milliohms.

ICODE is the 12-bit ADC code.

LSB25mV = 12.4 µV.

To convert a voltage to a 12-bit value, the following formula can

be used (round the result to the nearest integer):

VCODE = (VA/LSBxV) − 0.5

where:

VCODE is the 12-bit ADC code.

VA is the voltage value in volts.

LSBxV values are based on the voltage range (see Table 10).

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