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Data Sheet
Table 7. PMBus Conversion to Real-World Coefficients
Current (A)
ADM1075
Power (W)—Resistor Scaled
Coefficient
m
b
R
Voltage (V)
27,169
0
?1
ADM1075-1
806 × R SENSE
20,475
?1
ADM1075-2
404 × R SENSE
20,475
?1
ADM1075-1
8549 × R SENSE
0
?1
ADM1075-2
4279 × R SENSE
0
?1
Example 3
The READ_VIN command returns a direct format value of 1726.
The ADC_V pin is shorted to the OV pin, which is connected
to the input supply via an 820 k?/11 k? resistor divider.
To convert this value to the input voltage, use Equation 2
X = 1/27,169 × (1726 × 10 1 – 0)
X = 0.635 V
This corresponds to 0.635 V at the ADC_V pin. To obtain the
input voltage, this must be amplified by the resistor divider ratio,
X = 0.635 V × (820 k? + 11 k?)/11 k? = 47.99 V
Example 4
The PIN_OP_WARN_LIMIT command requires a power limit
value expressed in direct format.
If the required power limit is 350 W and the sense resistor is
1 mΩ, the first step is to determine the m coefficient. Assuming
an ADM1075-1 device, m = 8549 × 1 = 8549. The resistor
divider on VIN scales down the power limit referenced to the
ADC input. Assuming a 49 kΩ and 1 kΩ resistor divider on
VIN, this gives a scaling factor of 0.02.
Using Equation 1,
Y = (8549 × (350 × 0.02)) × 10 ?1
Y = 5984.3 = 5984 (rounded to the nearest integer)
Writing a value of 5984 with the PIN_OP_WARN_LIMIT
command sets an overpower warning at 350 W.
VOLTAGE AND CURRENT CONVERSION USING
LSB VALUES
The direct format voltage and current values returned by the
READ_VIN, READ_VAUX, and READ_IOUT commands, and
the corresponding peak versions are the actual data output
directly from the ADM1075 ADC. Because the voltages and
currents are 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, and 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
To convert an ADC code to current in amperes, the following
formulas can be used:
V SENSE = LSB xmV × ( I ADC ? 2048)
I OUT = V SENSE /( R SENSE × 0.001)
where:
V SENSE = (V SENSE+ ) ? (V SENSE? ).
LSB 25mV = 12.4 μV.
LSB 50mV = 24.77 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 INPUTV × ( V ADC + 0.5)
where:
V M is the measured value in volts.
V ADC is the 12-bit ADC code.
LSB INPUTV = 368 μV.
To convert a current in amperes to a 12-bit value, the following
formulas can be used (round the result to the nearest integer):
V SENSE = I A × R SENSE × 0.001
I CODE = 2048 + ( V SENSE / LSB xmV )
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.
LSB 50mV = 24.77 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 INPUTV ) ? 0.5
where:
V CODE is the 12-bit ADC code.
V A is the voltage value in volts.
LSB INPUTV = 368 μV.
method, with the exact LSB values, can provide slightly more
accurate numerical conversions.
Rev. C | Page 35 of 52
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