Calculations Related to PMBus* Specifications

Given the DIRECT format definition of $X=\frac{1}{m(Y\times {10}^{-R}-b)}$ :

Where:

• X is the calculated "real world" value in the appropriate units such as A, V, and °C
• m is the slope coefficient—a two-byte, two's complement integer
• Y is a two-byte, two's complement integer received from the PMBus* device
• b is the offset—a two-byte, two's complement integer
• R is the exponent—a one-byte, two's complement integer

You can determine the coefficients knowing that:

$X=\frac{Y}{4096}\times 2.5\text{V}\times \frac{(191\text{K}\Omega +100\text{K}\Omega )}{191\text{K}\Omega }$

$X=Y\times 9.29907\times {10}^{-4}$

$X=\frac{1}{1075.375945}\times Y$

Using the 16-bits resolution available for m, you get these constants:

$m=10753$

$b=0$

$R=-1$

Therefore, if you read back a value of 3549 after sending the READ_VOUT command, you can apply the constants to the formula and solve:

$X=\frac{1}{10753}(3549\times {10}^1-0)$

$X=3.300$