lm25066apsqx National Semiconductor Corporation, lm25066apsqx Datasheet - Page 43

• Manufacturer Part Number: lm25066apsqx
• Description: Lm25066a System Power Management And Protection Ic With Pmbus
• Manufacturer: National Semiconductor Corporation

Determining Telemetry Coefficients
Empirically with Linear Fit
The coefficients for telemetry measurements and warning
thresholds presented in Table 41 are adequate for the ma-
jority of applications. Current and power coefficients must be
calculated per application as they are dependent on the value
of the sense resistor, R
tions necessary for calculating the current and power coeffi-
cients for the general case. The small signal nature of the
current measurement make it and the power measurement
more susceptible to PCB parasitics than other telemetry
channels. This may cause slight variations in the optimum
coefficients (m, b, R) for converting from Direct format digital
values to real-world values (e.g., Amps and Watts). The op-
timum coefficients can be determined empirically for a spe-
cific application and PCB layout using two or more measure-
ments of the telemetry channel of interest. The current
coefficients can be determined using the following method:
1.
2.
3.
TABLE 43. Measurements for linear fit determination of
Measured voltage
While the LM25066A is in normal operation measure the
voltage across the sense resistor using kelvined test
points and a high accuracy DVM while controlling the
load current. Record the integer value returned by the
READ_AVG_IIN command (with the
SAMPLES_FOR_AVG set to a value greater than 0) for
two or more voltages across the sense resistor. For best
results, the individual READ_AVG_IIN measurements
should span nearly the full scale range of the current (For
example, voltage across R
Convert the measured voltages to currents by dividing
them by the value of R
R
resistor value of 5mΩ.
Using the spreadsheet or math program of your choice
determine the slope and the y-intercept of the returned
by the READ_AVG_IIN command values versus the
measured current. For the data shown in Table 42:
READ_AVG_IN value = slope x (Measured Current) + (y-
intercept)
slope = 683.4
y-intercept = -35.5
S
across
R
0.005
should be measured. Table 43 assumes a sense
0.01
0.02
S
(V)
current coefficients:
Measured Current
S
, used. Table 42 provides the equa-
S
. For best accuracy the value of
(A)
1
2
4
S
of 5mV and 20mV).
READ_AVG_IIN
(integer value)
1331
2698
648
43
4.
5.
Where:
X: the calculated "real world" value (volts, amps, watts, tem-
perature)
m: the slope coefficient, is the two byte, two's complement
integer
Y: a two byte two's complement integer received from device
b: the offset, a two byte, two's complement integer
R: the exponent, a one byte two's complement integer
The above procedure can be repeated to determine the co-
efficients of any telemetry channel simply by substituting
measured current for some other parameter (e.g., power,
voltage, etc.).
Writing Telemetry Data
There are several locations that will require writing data if their
optional usage is desired. Use the same coefficients previ-
ously calculated for your application, and apply them using
this method as prescribed by the PMBus™ revision section
7.2.2 "Sending a Value"
Where:
X: the calculated "real world" value (volts, amps, watts, tem-
perature)
m: the slope coefficient, is the two byte, two's complement
integer
Y: a two byte two's complement integer received from device
b: the offset, a two byte, two's complement integer
R: the exponent, a one byte two's complement integer
To determine the ‘m’ coefficient, simply shift the decimal
point of the calculated slope to arrive at at integer with a
suitable number of significant digits for accuracy
(typically 4) while staying with the range of -32768 to
+32767. This shift in the decimal point equates to the
‘R’ coefficient. For the slope value shown above, the
decimal point would be shifted to the right once hence
R = -1.
Once the ‘R’ coefficient has been determined, the ‘b’
coefficient is found by multiplying the y-intercept by
10
Calculated Current Coefficients:
m = 6834
b = -355
R = -1
-R
. In this case the value of b = -355.
Y = (mX + b) x 10
R
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