Due to the resistance of the connecting cable, and the resistances introduced for current sensing for overcurrent protection, the voltage at the hub can be anywhere between +4.5 V(loaded) and +5.5 V. This circuit will deliver a stabilized +5 V in both the cases, ie, it is a buck/boost design, using the TPS63000 switch mode regulator chip manufactured by Texas Instruments.
It can deliver +5 V at 500 mA from input voltages as low as 2 Volts so a rechargeable battery and its (USB powered) charger can be added to make this into a USB UPS for the USB hub.
Step 1: Preparing the Circuit Board
A scrap of single sided paper phenolic copper clad was cut to size and the outline of the chip drawn on its unclad side.
Then with a small screwdriver sharpened into a chisel, material was removed, making a niche for the chip to sit in.
Step 2: Gluing the Chip In
This is, strictly speaking, unnecessary but I liked the feel of gouging out the PCB material, and it was fun to add some three dimentionality to the circuit.
Step 3: The Ground Connections
Since the other side is an unbroken ground plane, this is easy: just drill holes and solder a wire.
Step 4: Drilling Holes
Step 5: Soldering Ground Leads
Step 6: Preparing the Inductor
I decided to dig the inductor in (for protection) so its shape has been marked on the board.
All this is, of course, really unnecessary.
Step 7: The Inductor
Step 8: The Hole for the Inductor
Step 9: The Inductor in Place
Step 10: The Input Filter
This makes the layout into a double sided board - sort of.
Step 11: The Output Connector and Capacitor
All the resistors and capacitors were rescued from junked hard disks.
Step 12: The Feedback Resistors
Ransacking all my surface mount boards (in my junkbox) threw up the pair you see in the figure. They were connected together as shown, then connected to a battery and my trusty multimeter verified that the division ratio was indeed ten.
If you are confused, on the left is a 523K resistor ie, 5, 2 and 3 followed by three zeroes, in ohms. On the right is a 4.7 Megohm resistor, ie, 4 and 7 followed by five zeroes, in ohms.
47 divided by nine is approximately 5.23.
Step 13: The Resistors in Place
The whole thing is held together with liberal applications of superglue - otherwise the solder joints might come apart each time the board fell off the table.
Now all that remains is for the inductor and input capacitor.
Step 14: Niche for the Capacitor, Too.
The outline of the capacitor has been marked on the board for cutting out.
Step 15: Capacitor Trench
Step 16: The Finished Board
It was tested.
First with two rather weak penlight cells - I did not trust my handiwork that much - and the output was 5.04 volts
Elated with the success, I tried it with three good cells - an input voltage of 4.5 volts - and the output was still 5.04 volts
Then I tried the voltage from the USB port of my computer - around 5 volts, though liable to jump around on the lower two digits - and still the output held steady at the same old 5.04 volts.
So it would seem that this thing works, at least during preliminary tests.
According to the datasheet it will start at 1.9 volts and accept a maximum of 5.5 volts, and hold its output voltage steady.
It is a buck - boost converter, which means it can accept input voltages above and below its output voltage, switching between modes automatically in order to keep the voltage steady.
It could be fed from a rechargeable cell in order to maintain the USB supply voltage even when the cable is disconnected from the computer - if that is any good.