Design Notes
Datasheet-Backed Component Choices
H-Bridge: NXP MC33926PNB
- Motor supply: VPWR class 5 V to 28 V; selected for 12 V nominal robotics motor supply.
- Supports bidirectional brushed DC motor control with IN1/IN2, D1/D2 disable, EN, INV, and SLEW inputs.
- Datasheet notes PWM up to 20 kHz; slow slew mode limits PWM below 11 kHz.
- Integrated protection: current limiting, short-circuit latch-off, overtemperature latch-off, undervoltage behavior, status flag.
- Required external support used in schematic plan:
- VPWR low-ESR bulk capacitance, 100 uF class, plus local 100 nF bypass.
- Charge-pump reservoir capacitor from CCP to VPWR, allowed 30 nF to 100 nF; 33 nF selected.
- SF active-low open-drain fault pull-up to 3.3 V.
- FB current mirror output support: 270 ohm to GND plus optional 1 uF filter if disable-PWM strategy is used.
MCU: STM32G431CBT6
- 3.3 V operation within datasheet VDD range 1.71 V to 3.6 V.
- Control-oriented Cortex-M4F with encoder-capable timers, PWM timers, ADC, USART/UART, FDCAN-capable pins, and SWD/JTAG.
- Power decoupling per datasheet:
- Each VDD/VSS pair gets 100 nF.
- VDD rail gets 4.7 uF bulk.
- VDDA/VSSA gets 10 nF + 1 uF filtering/decoupling.
- VREF+ gets 100 nF + 1 uF when used for ADC reference.
- BOOT0 on PB8 should be controlled; use pull-down for normal boot.
- NRST has embedded weak pull-up; add reset button and small reset capacitor for robust manual reset.
Current Sense: INA240A1D + 10 mOhm Shunt
- INA240 selected because datasheet explicitly targets PWM motor current sensing and supports -4 V to 80 V common-mode independent of supply.
- Gain: 20 V/V.
- Shunt: 10 mOhm inline with motor lead.
- Output bias: REF1 to 3.3 V and REF2 to GND gives 1.65 V zero-current midpoint.
- Current span:
- 2 A => 20 mV shunt => 0.4 V output swing, so output is about 1.25 V to 2.05 V.
- 5 A transient => 50 mV shunt => 1.0 V output swing, so output remains about 0.65 V to 2.65 V.
- INA240 supply decoupling: 100 nF close to VS/GND.
- Layout requirement: Kelvin route shunt sense traces.
Encoder Conditioning: SN74LVC3G17DCUR
- Triple non-inverting Schmitt buffer.
- VCC range 1.65 V to 5.5 V; powered from 3.3 V.
- Inputs accept up to 5.5 V at any valid VCC, useful for robotics encoders.
- 100 nF bypass cap required at VCC.
- All unused inputs must be tied to VCC or GND; all three channels are used for A, B, and Z.
3.3 V Buck: LMR33630BDDAR
- Selected instead of LDO for thermal efficiency from 12 V input.
- HSOIC/DDA package selected after ERC review to avoid the VQFN package's NC-to-SW pad conflict; datasheet pinout is PGND, VIN, EN, PG, FB, VCC, BOOT, SW, with thermal pad to GND.
- Datasheet 400 kHz 3.3 V component set used:
- L = 6.8 uH.
- RFBT = 100 kΩ, RFBB = 43.2 kΩ.
- CIN = 10 uF + 220 nF minimum; split/local ceramic capacitance recommended.
- COUT = 4 x 22 uF rated capacitance.
- CBOOT = 100 nF from BOOT to SW.
- CVCC = 1 uF from VCC to GND.
- EN tied to protected VM when always-on.
Reverse/Input Protection: LM74700-Q1 + DMT6008LFG + SMBJ33CA
- LM74700 ideal diode controller with external N-channel MOSFET for reverse polarity and reverse-current blocking.
- Datasheet recommends 60 V MOSFET, VGS max >= 15 V, and RDS(on) near 6.7 mΩ to 16.7 mΩ for a 3 A nominal design; DMT6008LFG 60 V, low-RDS(on) PowerDI MOSFET matches this class.
- VCAP: minimum 0.1 uF; selected 100 nF.
- Input capacitance: minimum 22 nF at ANODE; output capacitance minimum 100 nF, 47 uF typical holdup selected.
- SMBJ33CA bidirectional TVS selected for 12 V input transient protection.
Open Layout Checks
- Confirm MC33926 thermal pad copper area supports 2 A continuous in the selected enclosure/ambient.
- Size motor supply traces/pours for at least 2 A continuous and transient current.
- Place input TVS and ideal diode stage directly at the input connector.
- Place shunt in motor-current path and keep INA240 Kelvin traces short, symmetric, and away from switching nodes.