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# LTC2945 RSHNT formula:

Hi sir

For the bottom RSHNT formula, my customer replies to me and needs to know the RSHNT calculation method and application value.

(1)  Vs_max and VS_Min  , How Vs_max and VS_Min choose this value

(2)  -5.9 and -6.7 are fixed values that cannot be changed?

(3)  What do customers need to know about the maximum current and maximum PD on the RSHNT path?

(4)  Customer condition as below

If Vs(max) = 5.1V, Vs(min)=4.9V, lLoad(max)=1A.

Is this calculated value correct?

Nested formula, -23 ohm < Rshunt < -1.8 ohm.
Is that right?

P4 RSHNT formula

https://www.analog.com/media/en/dsp-documentation/evaluation-kit-manuals/dc1697afa.pdf • Greetings Jason,

(1)  Vs_max and VS_Min  , How Vs_max and VS_Min choose this value

The Vs max and min values are the operating min/max of the supply voltage

(2)  -5.9 and -6.7 are fixed values that cannot be changed?

Correct.  These values correspond to the shunt regulator min/max voltages

(3)  What do customers need to know about the maximum current and maximum PD on the RSHNT path?

When Rshunt is used, the maximum power dissipation should be taken into consideration and the shunt resistor rated accordingly. Current to IntVcc should be limited to below 35mA.

(4)  Customer condition as below

If Vs(max) = 5.1V, Vs(min)=4.9V, lLoad(max)=1A.

Is this calculated value correct?

With a presumed supply voltage of 5V (based off of provided min/max) the shunt is not necessary as it can be connected directly to the INTVCC pin.  When powered this way, you should connect VDD and INTVCC to minimize on-chip power dissipation The external shunt is needed when supply voltage is >80V.

Nested formula, -23 ohm < Rshunt < -1.8 ohm.
Is that right?

Based on the values you provided, the math is correct but not really applicable at such low voltage.

Hopefully this helps answer your questions!  If not, let me know and I will do what I can to help.

Best regards,

• Hi Sir

I still have some questions as follows:

(1) Customers need 20A on the Rshunt path, can this current be reached?
Customers currently use 5m ohms of Rshunt to reach 20A demand
I need to know that current values and wattage circuits are affordable.

(2) So I use the DATASHEET P28 Rshunt formula to calculate
Nested formula, -23 ohm < Rshunt < -1.8 ohm. This Rshunt design value
But you said that the calculation is correct for the Rshunt value, but the Rshunt value he calculated is falling on the answer       as above.

(3) It is known by the circuit that it can output 80V, but what is the maximum application current value of 80V?(Iout_Max)=?
Is Rshunt to determine my output power? I know by the specification that I can output power of 80VIN/5A?

• Greetings Jason,

I believe that there is some minor confusion regarding RSHUNT vs RSNS.  The RSHUNT that you are calculating a value for is not the sense resistor value. It is a way of powering the chip through the internal shunt regulator when the supply voltage is >80V:

"For supply voltages above 80V, the shunt regulator at INTVCC can be used in both high and low side configurations to provide power to the LTC2945 through an external shunt resistor, RSHUNT" The LTC2945 measures current indirectly by measuring the voltage difference across the RSNS resistor. The customer's use of a 5mOhm RSNS resistor allows for a max current of 20.48A to be measured (.1024VFS / .005Ohm).  If the customer requires a higher max current to be read, then they should use a smaller RSNS value.  The limiting factor as to how much current they can measure is power dissipation.  A 5mOhm RSNS resistor at 20A would need to be rated for at least 2W.  The datasheet offers some helpful layout recommendations which will help keep board temperature under control.  Let me know if you have any questions!

Thank you and best regards,

• Hi Sir

Thanks for you support

Is there an RSNS formula for reference?

I need to discuss with the customer how the value of RSNS is calculated.

• Greetings Jason,

To calculate RSNS you will need the full scale voltage for ΔVsense (VSENSE+ - VSENSE-) from the datasheet and the maximum current the customer would want to read.  In this case, the LTC2945 has a ΔVsense of 102.4mV and the current is 20A.  Using Ohm's Law:

RSNS = 0.1024V / 20A = 0.00512Ω

To calculate the LSB weight, take the LTC2945 step size of 25uV/LSB from the datasheet and divide by the RSENSE value.  Using the RSNS value calculated above:

Resolution = 0.000025V / 0.00512 =  0.00488A

Hope this helps!

Thank you and best regards,