1.9 KiB
Jacdac power
JD device that delivers power to the bus needs to limit it. This document has notes on one way of implementing this.
In both options:
- add FET high-side switch on power delivery to the bus
- probe current every 1ms, with slight randomization (eg. every 900-1100us)
Cheap option
- add shunt resistor on the low side of power line (GND)
- connect bus side of shunt to ADC via 3/5 voltage divider
Assuming 12-bit ADC with reference voltage of 3.3V the table below gives precision of measurement per ADC step and voltage drop at 500mA.
| Shunt | Vdrop | Precision | Size |
|---|---|---|---|
| 0.50Ω | 5.0% | 2.7mA | 0603 |
| 0.36Ω | 3.6% | 3.7mA | 0402 |
| 0.22Ω | 2.2% | 6.1mA | 0402 |
| 0.20Ω | 2.0% | 6.7mA | 0402 |
Precise option
- add INA199A1 current sensing amplifier (50V/V)
- add shunt on either power or GND side
| Shunt | Vdrop | Precision | Max Curr | Size |
|---|---|---|---|---|
| 0.10Ω | 1.0% | 0.161mA | 660mA | 0603 |
| 0.05Ω | 0.5% | 0.322mA | 1320mA | 0805 |
| 0.02Ω | 0.2% | 0.806mA | 3300mA | 0805 |
Additionally, can add additional switching FET and another shunt for precision measurements, when current is low. Voltage drops listed for 50mA.
| Shunt | Vdrop | Precision | Max Curr | Size |
|---|---|---|---|---|
| 1.00Ω | 1.0% | 0.016mA | 66mA | 0402 |
| 0.50Ω | 0.5% | 0.032mA | 132mA | 0603 |
Low pass filters
With 100Hz RC filter, if the current goes from max to 0, the voltage will drop by ~45% in 1ms. This represents a measure of error (the current we may miss when not sampling). OTOH, it takes 2ms for voltage to reach 70% when it goes from 0 to max, which is a measure of latency.
With 50Hz, it drops by ~25% in 1ms and it takes 4ms to register 70% of rise.
With 25Hz, it drops by ~15% in 1ms and it takes 8ms to register 70% of rise.
50Hz is eg. 3.3kΩ and 1uF.