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參數(shù)資料
| 型號(hào)： | LM25066PSQX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 電源管理 |
| 英文描述： | POWER SUPPLY SUPPORT CKT, QCC24 |
| 封裝： | LLP-24 |
| 文件頁數(shù)： | 37/48頁 |
| 文件大?。?/td> | 1456K |
| 代理商： | LM25066PSQX |
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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
S, used. Table 42 provides the equa-
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 opti-
mum coefficients can be determined empirically for a specific
application and PCB layout using two or more measurements
of the telemetry channel of interest. The current coefficients
can be determined using the following method:
1.
While the LM25066 is in normal operation, measure the
voltage across the sense resistor using kelvin 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
S of 5 mV and 20 mV).
2.
Convert the measured voltages to currents by dividing
them by the value of R
S. For best accuracy, the value of
R
S should be measured. Table 43 assumes a sense
resistor value of 5 m
.
TABLE 43. Measurements for linear fit determination of
current coefficients:
Measured voltage
across
R
S (V)
Measured Current
(A)
READ_AVG_IIN
(integer value)
0.005
1
648
0.01
2
1331
0.02
4
2698
3.
Using the spreadsheet or math program of your choice,
determine the slope and the y-intercept values returned
by the READ_AVG_IIN command versus the measured
current. For the data shown in Table 43:
READ_AVG_IIN value = slope x (Measured Current) +
(y-intercept)
slope = 683.4
y-intercept = -35.5
4.
To determine the ‘m’ coefficient, simply shift the decimal
point of the calculated slope to arrive at an 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.
5.
Once the ‘R’ coefficient has been determined, the ‘b’
coefficient is found by multiplying the y-intercept by
10-R. In this case the value of b = -355.
Calculated Current Coefficients:
m
= 6834
b
= -355
R
= -1
where:
X:
the calculated "real world" value (volts, amps, watts, tem-
perature)
m:
the slope coefficient, a the two byte two's complement in-
teger
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, volt-
age, 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"
Y = (mX + b) x 10R
where:
X:
the calculated "real world" value (volts, amps, watts, tem-
perature)
m:
the slope coefficient, a 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
www.national.com
42
LM25066
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