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Data Sheet
Table 9. PMBus Conversion to Real-World Coefficients
Voltage (V)
ADM1275
Coefficient
m
b
R
Current (A)
807 × R SENSE
20,475
?1
0 V to 6 V Range
6720
0
?1
0 V to 20 V Range
19,199
0
?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 m?.
To convert this value to the current flowing, use Equation 2, with
m = 807 × 1.
X = 1/807 × (3339 × 10 1 ? 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:
V SENSE = LSB 25mV × ( I ADC ? 2048)
I OUT = V SENSE /( R SENSE × 0.001)
where:
V SENSE = (V SENSE+ ) ? (V SENSE? ).
LSB 25mV = 12.4 μV.
I ADC is the 12-bit ADC code.
I OUT is the measured current value in amperes.
R SENSE is the value of the sense resistor in milliohms.
To convert an ADC code to a voltage, the following formula can
be used:
V M = LSB xV × ( V ADC + 0.5)
where:
V M is the measured value in volts.
V ADC is the 12-bit ADC code.
Table 10. Voltage Ranges and LSB Values
Voltage Range, LSB xV 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):
V SENSE = I A × R SENSE × 0.001
I CODE = 2048 + ( V SENSE / LSB 25mV )
where:
V SENSE = (V SENSE+ ) ? (V SENSE? ).
I A is the current value in amperes.
R SENSE is the value of the sense resistor in milliohms.
I CODE is the 12-bit ADC code.
LSB 25mV = 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):
V CODE = ( V A / LSB xV ) ? 0.5
where:
V CODE is the 12-bit ADC code.
V A is the voltage value in volts.
Rev. D | Page 33 of 48
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