Soft Latching Power Switch Circuit
The design of the soft latching power switch required quite some time, I search the internet extensively to find a good inspiration, but almost all the circuits were designed for 5V or 12V power supply and adapting them for the required working voltage range (9 to 24V) was often not feasible or impossible. I tried the omnipresent EEVBlog solution, but I wasn't pleased with its performance. Other circuits couldn't work with a large capacitance as load, and others didn't work at all.
So as the first iteration, I decided to use a specialized IC solution using the MAX16054 chip and clamping its working voltage with a zener. (here below the solution)
Unfortunately, due to the input supply range, the lowest current consumption in OFF state was around 200uA; I could improve it with the new series nanoPower MAX16150, but the chip lacks a SOT case version.
The MAX16054 solution was a decent starting point, but I wanted to develop a reliable switch with virtually no standby current consumption, to use eventually on other projects without any worries of OFF state consumption, and input voltage. In the end, after more extensive research, I stumbled in this circuit:
Frankly, I do not know who the author is, I found it among a bunch of other samples, but the idea was intriguing and worth to be built and tested. To optimize the values of the components and to improve the switch stability I developed a PSpice simulation using Microcap 12 (many thanks to Spectrum Software for making it free software) Below the circuit in his final design:
It is essential to use the exact PSpice model of the P-Mosfet to have an accurate simulation.
The Mosfet is a little oversized for the application, but the idea was to have a standard circuit for different applications, and a 25mΩ RDSon resistance is always a good thing.
I added the JP1, wich shorts U1A to keeps the switch always ON. The capacitor C5 adds some stability at the power-up and improves the overall stability of the switch, and the value can't vary very much; otherwise, the system will stop working. To reduce the required PCB space, I used a complementary transistor couple in a single package.
I used two components to protect the inputs from transients that can be present in a typical alternator source power supply: TVS1 with a standoff voltage of 26V and a clamping voltage of 42V and a P-Mosfet Q1 to protect the input from a reverse voltage miswiring.
Raspberry Power Supply
To provide a clean and stable power supply to the Raspberry, I used a Simple Switcher from Texas Instrument. Since the goal is to have a low ripple and clean power supply, I choose to use the chip with the FPWM option.
The output and input capacitors are ceramic to improve transient response, and the component placement follows the datasheet's suggestions. I added a 51pF capacitor to increase the stability of the feedback loop. The feedback point has been taken after the current sense resistor of the UPS to improve voltage stability on high load.
The overall ripple at 2A is about 20mVpp.
In theory, the power supply can deliver up to 3A, but due to the constraint of the copper available, the maximum load is around 2,5A, the case temperature of chip settles around 110ºC.
For the average consumption of the Raspberry and some external load, the continuous load is around 1.5A, and the chip settles at 67ºC.
To prevent the power supply to work with an input voltage too low, a voltage divider based on R6 and R7 is used to set a minimum start working voltage of 9.5V and a shutoff of 7V.
The SuperCapacitor UPS
All the UPS functions are performed by the LTC4041, which include a supercapacitor charger and a boost converter, plus all the ancillary circuits to make it a complete solution. The circuit is quite similar to the application note, so I will not spend time describing the inner working of the chip, but only the additions to...Read more »