Routing preflight found C2 and D1 inside the bottom-left mounting-hole keepout. Routing cannot preserve both the approved placement and the required Ø8 mm mechanical clearance; moving those two parts is the recommended fix.
The autorouter connected 125 of 134 initial airwires, and surgical routing reduced the remainder to six. The remaining pins are physically blocked by the approved placement/existing copper, so further retries without changing placement or deleting working routes would not converge; I’m running the final DRC summary now.
Routing is substantially complete: 753 trace segments and 154 vias were added, with no overlapping-copper, via-in-pad, dangling-trace, or redundant-trace errors.
Open items
6 airwires remain:
EFUSE_OVP: U2 OVP → R5
3V3: U4 VOS → existing 3V3 via
Four ground-plane connections around U6, C13, and R3
These remained blocked after two targeted attempts; finishing them requires local copper rip-up and likely small placement adjustments.
Non-blocking placement advisories remain for five decoupling loops and the U3→L1 switch-path detour.
Suggested next steps
Authorize surgical movement around U2/U4/U6 and local trace rip-up to clear the six airwires.
Re-run full DRC and power-integrity checks.
Reconcile the routed PCB against the frame and USB-C cable clearance.
The schematic has been arranged automatically. Flux can redraw it to be more readable, with components placed in related groups and wires drawn more legibly. This takes about 10-60 mins.
Feasibility / Design Notes
Feasibility Position
The concept is electrically plausible within a 9 V/3 A (27 W) PD budget, but stable tethered flight is conditional, not yet proven. The next phase must bench-test the motor/prop/tether combination before freezing enclosure geometry or power components.
Candidate Motor / Prop Class
Use an 8.5 × 20 mm brushed coreless motor class with approximately 55 mm CW/CCW propellers as the baseline prototype direction. Public vendor data confirms this class is used on roughly 80–110 mm micro quad frames and is available in 1S and 2S windings, but published current and thrust data are inconsistent. Therefore this document defines acceptance targets rather than claiming a specific unverified motor performance.
Constar 8520 motor family: multiple 3/5/7 V windings and materially different resistance/current curves, demonstrating that “8520” identifies size, not one electrical design: http://www.constarmotor.com/productinfo/6544.html
Conclusion: buy at least two candidate matched motor/prop sets and characterize them. Do not select MOSFETs, converter current limit, pod geometry, or control feed-forward values solely from the size code.
Mass and Thrust Estimate
Provisional flying-mass target:
Table
Item
Estimate
4 motors
16–20 g
PCB + components + connector
12–18 g
skeletal printed frame + guards
18–25 g
local tether pigtail/strain relief + wiring
5–10 g
fasteners/adhesive margin
3–5 g
Estimated airborne assembly
54–78 g
Design to ≤75 g assembly mass. The operator must support excess tether; the vehicle is not expected to lift the full cable length.
For 75 g mass:
Hover thrust = 75 g total = 18.75 g per motor ideally balanced.
Minimum 1.5× static thrust margin = 112.5 g total = 28.1 g per motor.
Preferred 1.7× margin = 127.5 g total = 31.9 g per motor.
Prototype motor/prop acceptance target: ≥28 g static thrust per motor at ≤1.25 A on the selected motor rail, with a preferred result near 32 g/motor. If this is not met, reduce mass, test a different winding/prop, or revise the source/power architecture; do not hide the deficit in control tuning.
Power Budget
Rail assumptions
PD input: 9 V nominal, 3 A source limit, 27 W maximum.
Motor rail: adjustable around 4.0 V for initial 1S-class testing; aggregate firmware/current cap 5.0 A continuous.
Motor buck efficiency: 88% at high load (conservative design estimate pending component selection).
Logic rail: 3.3 V buck, 90% efficiency, 70 mA design peak.
PD sink/control allowance: 10 mA from 9 V.
Load table
Table
Load
Hover assumption
Peak/design assumption
Motor 1
0.8 A @ 4.0 V
allocated within aggregate cap
Motor 2
0.8 A @ 4.0 V
allocated within aggregate cap
Motor 3
0.8 A @ 4.0 V
allocated within aggregate cap
Motor 4
0.8 A @ 4.0 V
allocated within aggregate cap
Four motors total
3.2 A / 12.8 W
5.0 A / 20.0 W continuous
MCU
25 mA @ 3.3 V
40 mA @ 3.3 V
IMU
5 mA @ 3.3 V
10 mA @ 3.3 V
LED + sensing + pullups
10 mA @ 3.3 V
20 mA @ 3.3 V
3.3 V total
40 mA / 0.132 W
70 mA / 0.231 W
Input reflection
Hover motor input:
12.8 W / 0.88 / 9 V = 1.62 A
Design-cap motor input:
20.0 W / 0.88 / 9 V = 2.53 A
3.3 V rail at peak:
(3.3 V × 0.070 A) / 0.90 / 9 V = 0.029 A
PD/control allowance:
9 V × 0.010 A = 0.09 W = 0.010 A at input
Total calculated design input:
2.53 + 0.029 + 0.010 = 2.57 A
Remaining source margin:
3.00 - 2.57 = 0.43 A, or approximately 3.9 W at 9 V.
This margin is modest. Simultaneous motor stall is outside the supported operating case and must cause current limiting/disarm rather than sustained delivery. A short transient above 5.0 A motor current may be evaluated only after measuring PD cable droop, converter response, and motor thrust benefit.
Power-Path Sizing Inputs
USB-C receptacle/tether conductors, PD path, protection, and PCB copper: 3 A continuous minimum at 9 V with voltage-drop review.
Motor buck: 4.0 V adjustable, 5.0 A continuous output minimum, transient capacity and thermal design subject to bench test.
Motor driver channel: select only after measured locked-rotor current; target low RDS(on), logic-level gate drive, and repetitive PWM capability with margin above measured single-motor stall current.
Logic buck: 3.3 V, ≥150 mA.
Input and motor-rail capacitance: determine from converter datasheet and measured tether impedance; do not rely on large capacitance to mask an inadequate cable or unstable PD path.
Avoid a resettable fuse chosen by nominal current alone; its resistance and hot derating could consume critical headroom. Use controller current limiting and/or a suitably low-loss protection strategy after detailed selection.
Bench Validation Plan Before CAD/Electronics Freeze
Measure each candidate motor/prop at several rail voltages: current, static thrust, RPM if available, and winding temperature.
Record hover-equivalent thrust/current and maximum safe 10–20 second thrust/current.
Measure locked-rotor current with a current-limited supply using a brief safe test, or obtain credible manufacturer data.
Weigh motor, prop, guard test piece, proposed tether per meter, and strain-relief parts.
Hang representative tether lengths from a force gauge to quantify vertical load and lateral restoring force versus displacement.
Test cable torsion by rotating the attachment about yaw and recording restoring moment qualitatively or with a small lever arm.
Build a single-arm vibration sample to check IMU vibration transmission and motor retention.
Freeze motor pod, guard, converter, MOSFET, and cable ratings only after these results.
Key Risks
Table
Risk
Severity
Mitigation / decision gate
Insufficient thrust margin after guards/tether
Error if <1.5×
Validate ≥28 g/motor at ≤1.25 A; reduce mass or change motor/prop
Tether bending/torsional moment overwhelms control
High
Very flexible cable, center attachment, supported slack, tether-force tests
9 V PD dropout from cable/transients
High
Dedicated source, low-loss cable/path, input telemetry, bulk capacitance per converter design, aggregate current cap
Weigh early prints; skeletal geometry; mass budget gate before full CAD
Converter/MOSFET heating
High
Thermal calculations after part selection and supervised 2-minute full-load test
Ground effect and tether contact during takeoff
Medium
Short controlled spool-up, central tether guide, low stable rest geometry
Vendor “8520” ambiguity
High
Treat size only as a mechanical class; validate exact purchased MPN/sample
Control tuning instability
High
Tethered test stand, duty/current limits, rapid disarm, staged axis tuning
Go / No-Go Gates
Go to enclosure CAD: exact motor/prop samples chosen; measured swept diameter and mounting geometry known; tether sample chosen; mass budget predicts ≤75 g and ≥1.5× thrust margin.
Go to schematic capture: exact motor operating/stall data, motor rail voltage/current cap, USB-C PD sink approach, and converter architecture are frozen sufficiently to rate the power path.
No-go: any candidate requires more than the 9 V/3 A source budget for the minimum thrust margin, or the tether produces unmanageable moments in a supported-slack bench test.
Electrical Capture Status
The schematic is now captured around HUSB238A PD negotiation, TPS25940 input protection, TPS56637 motor conversion, TPS62162 logic conversion, STM32G431 control, ICM-42688-P sensing, and four AO3400A/SS34 brushed-motor channels. The motor rail is configured near 3.97 V and defaults disabled in hardware. The implemented eFuse current-limit resistor targets approximately 3.18 A input current.
The electrical architecture is ready for PCB placement and bench validation, but the overall vehicle remains conditional on the original motor/prop/tether tests. The AO3400A and SS34 selections must be rechecked against measured locked-rotor current and commutation transients before sustained propulsion testing. Converter thermal performance and PD cable droop also remain physical-test gates.
Feasibility Position
Candidate Motor / Prop Class
Mass and Thrust Estimate
Power Budget
Rail assumptions
Load table
Input reflection
Power-Path Sizing Inputs
Bench Validation Plan Before CAD/Electronics Freeze