Datasheet

X =
1
m
(Y x 10
-R
- b)
LM5066
SNVS655G –JUNE 2011–REVISED FEBRUARY 2013 www.ti.com
DETERMINING TELEMETRY COEFFICIENTS EMPIRICALLY WITH LINEAR FIT
The coefficients for telemetry measurements and warning thresholds presented in Table 41 are adequate for the
majority 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 equations necessary for calculating the
current and power coefficients 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 (for example, Amps and Watts). The optimum 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 LM5066 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 5mV and 20mV).
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 5mΩ.
Table 43. Measurements for linear fit determination of current coefficients:
Measured voltage across Measured Current (A) READ_AVG_IIN
R
S
(V) (integer value)
0.005 1 568
0.01 2 1108
0.02 4 2185
1. Using the spreadsheet or math program of your choice determine the slope and the y-intercept of the data
returned by the READ_AVG_IIN command versus the measured current. For the data shown in Table 42:
– READ_AVG_IN value = slope x (Measured Current) + (y-intercept)
– slope = 538.9
– y-intercept = 29.5
2. 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.
3. 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 = 295.
– Calculated Current Coefficients:
– m = 5389
– b = 295
– R = -1
(53)
Where:
X: the calculated "real-world" value (volts, amps, watts, temperature)
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 coefficients of any telemetry channel simply by
substituting measured current for some other parameter (for example, power, voltage, etc.).
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