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# LTC2945 register Question

1. All from 5h to dh are 0x00. (Even though bh, ch, dh default to 0xff) Is there any value that should be initial to monitor?
How does the power calculation work? I would appreciate an example.

2. What are the criteria for updating ∆SENSE MSB/LSB (14h, 15h) and MAX/MIN ∆SENSE MSB/LSB (16h to 19h)? There is no difference in value even if the input voltage is changed. (I tried changing 40~50V) (MIN is always 0.)

• Greetings Parksh,

Great questions. I'm sorry to hear you are having problems, but I think I can point you in the right direction.  I suspect, due to min registers reading 0x00, that you may have SENSE- above SENSE+.  The LTC2945 does not read current bidirectionally so the ADC will read the result as 0x00 and update the power and current registers accordingly, including changing the min power values from 0xFF to 0x00.

Calculating the power is a topic that comes up fairly regularly.  The LTC2945 has no knowledge of the value of the sense resistor being used, so the calculation must be performed manually or in software.  The datasheet lists the delta Sense LSB size as 25uV. Using this information, along with a known sense resistor value and Ohms law, allows us to solve for the current (I = V/R).  Assuming a 5mOhm sense resistor, we get a current per LSB of:

0.000025V / 0.005Ohm = 0.005A

so with a 5mOhm sense resistor, each LSB of current in the delta sense register equates to 5mA of current.  Calculate the power, P=IV, the same way.  1LSB of voltage = 25mV, 1LSB of current = 5mA (based on the 5mOhm Res).  1LSB of power would be:

0.025V * 0.005A = 0.000125W

Multiply these values by what resides in the appropriate register to get the total value.

I hope that this helps!

Thank you and best regards,