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3V DotBot Design Journal

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Said Alvarado edited this page Feb 10, 2024 · 6 revisions

Here we document the calculations and progress of the redesign of the DotBot to work with 3v motors.

6v Step Up --> 5v Step up

The motors will now be connected directly to the batteries, there is no need to have a high power 6v step-up. A 5v step up is still necessary to power the RGB Led and other 5v sensors. We will repurpose the 3.8v step up for this purpose.

Changes

  • Remove LM5158 and all support circuitry.
  • Change MAX1723 (3.8v step-up, adjustable) for a MAX1724EZK50+T (5v step-up, fixed).
  • Re-route all 6v traces to the new 5v regulator.

Requirements

  • nRF GPIO Voltage: 1.8V ~ 3.2V must be able to shut down the regulator
  • Regulator must power all 3.3v and 5v digital sensor and LEDs on the board, about 50mA.
  • Input voltage varies between 3.2V and 1.8V as the battery depletes, Regulator must work throughout this range.

MAX1724 Calculations

datasheet.

  • ✅ VIL / VIH for the Shutdown pin: 70mV / 800mV
  • ✅ Min Input voltage: 0.8V
  • ✅ Min Start-up Voltage: 0.8v
  • ✅ Max Ouput Current: 150mA

New Low-voltage, High-current Motor driver

DRV8411A Dual H-Bridge Motor Driver with Current Regulation.

Compatibility

  • Dual Full H-Bridge
  • Pin compatible with the previous Motor Driver DRV8833.
    • We will use the QFN-1 package
  • Also has current sense and current regulation.
  • SLEEP is activated by setting all inputs to LOW.
    • No need for an extra pin.
  • VIH/VIL compatible with 1.8v logic.
  • Usable, and communicable, throughout the entire voltage range. (1.8v to 3.3v)
  • Roughly same price as DRV8833
    • QFN 2.26$/u (0.78$ @ 5ku).
    • TSSOP 1.33$/u (0.84$ @ 3ku)

Calculations

VREF has to be less than VM. VREF is the highest voltage that will appear at the AIPROPI and BIPROPI pins. SO the lower, the nRF ADC can't handle more than VDD at the Analog pins. Then we decide:

  • VREF = 0.6v
  • Max Current per motor = 500mA Ripropi can then be calculated from equation (3) in the Datasheet.

VREF must be stable, so we derive it from a voltage divider at the LDO 3.3v line.

Closest achievable values with standard value resistor:

  • VREF_R1 = 1M

  • VREF_R2 = 220k

  • VREF = 0.595v

  • R_IPROPI = 6.1k

  • Max Current per motor = 487mA

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