Schematic Review - 07/02/2022

Owned by Paul Kokhanov
Last updated: Aug 18, 2022 by Rishith Bomman
6 min read

https://github.com/uwrobotics/MarsRoverHardware/tree/master/Projects/24V%20Multiphase%20Buck/Rev%202

Datasheet Calculations

Formulas used: Datasheet Calculations.docx

Calculations used: https://uofwaterloo-my.sharepoint.com/:x:/g/personal/laphartf_uwaterloo_ca/EXZmLZJsgfhKl3uAZ2ZXCosBsq3ntQcYzEnib-hCZegW1w?e=pE0gwM

Reviewer Name

Manufacturer Part Number

Symbol Name

Footprint Name

Comments

Reviewer Name

Manufacturer Part Number

Symbol Name

Footprint Name

Comments

Ari

IPB073N15N5ATMA1

NMOS_150V_114A

 NMOS_DPAK263-3-2

 

24V Multiphase Buck.SchDoc (Rishith)

All the calculations appear to be correct, no issues there

Not quite sure why there is a x4 in the Pmax on the single FET Calculation

Not quite sure why there is a x2 in the Cin Calculation

Not quite sure why the Phs Conducting calculation has the Imax divided by 2

Unless I’m missing something, the Phs switching doesn’t seem to include fsw anywhere, I’m not sure what is going on in this calculation.

Circuit itself matches the datasheet and previous version, all the components are in the right spot

In the typical applications, SS1 has a resistor + capacitor, meanwhile yours only has a capacitor

What is the purpose of the 0 Ohm resistors, is there a reason you don’t just use a regular wire?

Shouldn’t there be a capacitor between your Cs1- and Cs1+ rails?

Shouldn’t there be a capacitor between your Cs2- and Cs2+ rails?

Component values match those determined in the calculations

Comments:

 

is there a reason that the decoupling capacitors are connected to the current sensing port, or can this be separated on the page

24V Multiphase Buck.SchDoc (Josh)

All the calculations appear to be correct, no issues there
Circuit itself matches the datasheet and previous version, all the components are in the right spot
Component values match those determined in the calculations

Comments:

24V Multiphase Buck.SchDoc (Ari)

Please focus on the Peak Current Limit Programming and Current Sensing sections of the datasheet calculations

All the calculations appear to be correct, no issues there
Circuit itself matches the datasheet and previous version, all the components are in the right spot
Component values match those determined in the calculations

Comments:

 

For current sensing calculations, I got 0.0026 Ohms, but i’m not sure if that's correct.

should we be calculating the MIN or the MAX for this equation? Because RSENSE takes the m

ax. Also what VCS value are you supposed to use. on the calculation sheet, there is a min and max

 

Response: We are not too concerned with having an exact value for Rsense, we decided to use 0.005 Ohm as it it an easier resistor value to work with and won’t affect the further calculations as drastically

COMP1 and COMP2 values for resistor and capacitors don't match calculations in the calculation sheet

 

Response: These were updated on the new schematic.

 

PIN2 RRT Value might be wrong. I calculated (295693+133)/8.8 = 33601 ohms

 

Response: Yes your suppose to use 133000

 

This only regulates the output. it will shut off if less than 24 volts. don't you want to regulate the input?

 

24V Multiphase Buck.SchDoc (Farris)

All the calculations appear to be correct, no issues there
Circuit itself matches the datasheet and previous version, all the components are in the right spot

Comments on component values and part selection:

  • For your inductor, I would reduce the peak-to-peak current ripple to 20% instead of 30%, due to how much current is going through the inductor. This will help reduce losses from the inductor. I’d also select an inductance that is equal to or higher than the calculated inductance value, instead of below

  • IC Internal Power Dissipation: Note that for this equation, Qgate is the sum of the gate charge for BOTH high and low-side FETs of BOTH controllers, in other words every single FET’s gate charge is summed up in this equation. This means 8 total FETs for a double FET config, and 4 total FETs for a single FET config.

    • Currently, you’re counting only 4 total FETs for a double FET config and 2 total FETs for a single FET config, so your calculated values for power dissipation are half of the actual values

    • It also means that currently your internal power dissipation is way too high, even for the single FET config. Couple ways to solve this:

      • Reduce your switching frequency, maybe down to 200kHz. This reduces the power dissipation by almost 45%

        • Be careful of the other component values that this will change, mostly just a bunch of resistors and ceramic capacitors that will need adjusting

      • Find a FET with lower gate charge (even just slightly lower will help a lot as it gets compounded 4 or 8 times)

      • Reduce the voltage at the VCCEXT pin (see comment under general comments)

    • We should aim to keep the internal power dissipation at 2W or lower

  • For your bootstrap capacitors, remember you need to sum the gate charge for 2 FETs, not just one, if going with the double-FET config

General comments:

  • Careful, the GND port on R2 isn’t connected properly; Altium currently sees it as floating

  • Seems the current limit was reduced from 17.5A to 15.5A, why is this?

  • I recommend adding a voltage divider to the 24V on the VCCEXT pin, as with the ripple on the 24V it will go over the 24V limit on the VCCEXT pin. I recommend dividing the voltage by half, as the minimum is only 4.7V

    • Lowering the VCCEXT voltage also greatly reduces the IC’s internal power dissipation

Net Naming (Farris)

Wiring and Connections (Farris)

Component Labels (Farris)

Design-for-Test (Farris)