Jason has been working on a portable software defined transceiver design for the past year. Every day this week he’ll discuss a different part of the hardware in a series of guest posts. You can chat with the designer in the forum. Today’s post is about the power supply:
It’s not as exciting (to me) as the other parts of this design, but it’s very important for maximizing the length of time this transceiver can run on batteries. The transceiver uses four different voltages, 12V, -12V, 5V, and 3.3V. The batteries will provide 12V, so the other three voltages need to be generated on the board. For the 3.3V and 5V supplies, I’ve selected the LM2674 switcher for both voltages (there are 3.3V and 5V versions). It provides up to 500mA, and according to their design software, will yield an efficiency of between 85-87%.
Lots more below.
For the 12V supply, I’m going to use the TC7662 charge pump. It can’t provide a lot of current, but I don’t need much at -12V, and the efficiency is very high at 97% or thereabouts. The LM2674 has a switching frequency of 260 KHz and the TC7662 has a switching frequency of 35KHz (with the boost pin tied high). Both of these frequencies are well above my passband (20KHz) and well below any of the HF bands it’s designed to operate on. Furthermore, the op amps I’m using in the IF are designed to have good PSRR figures which should also minimize the effect of ripple on any power lines.
There is plenty of excess current capability for driving things like touchscreen displays or other UI devices which may be used in the future.
Here is the schematic, and a high-resolution PDF version is attached as well for anyone interested
Via the forum.