Every engineer has a favourite part. Every so often, an IC comes along that is so useful, you wonder what you did before it came along. You use it wherever you can and you use it everywhere.
There are some great examples: The ubiquitous 555 timer; the 741 op-amp; and so on. Of course, you use the 7555 CMOS version because it is lower power and doesn’t have any power supply crowbar problem and the NE5532 low noise amplifier, but basic topology is the same.
With power supply components, the LM317 adjustable linear regulator springs to mind. But what about switching power supplies? Pretty much every switching regulator is proprietary, so it’s a little more difficult.
Well, my favourite part is the PI3749 ZVS Buck-Boost regulator. With an input voltage range of 16 to 34V and output range of 12 to 28V, the Buck-Boost architecture means the output can be either above or below the output voltage. The output voltage is adjusted by feeding back a portion of the desired output through a voltage divider to the error amplifier’s input.
There are a couple of ways of doing this. One is to use a digital potentiometer. The top resistor in the feedback chain is the one that needs to be changed, so the digi-pot needs to be able to support almost the entire output voltage. Most are designed for 5V supplies but there are some high-voltage ones from Microchip that fit the bill: The MCP45HVxx I2C version and MCP41HVxx SPI version.
An alternative, one that is implemented on the evaluation board, is to have fixed resistors for R1 and R2 to set the nominal trim and then to add a third resistor to the error amplifier node. Kirchhoff’s Current Law stated simply says that all currents flowing into a node must sum to zero. So, by using a DAC to apply a voltage to the other end of this resistor, we can add current to or subcontract current from the error amplifier node, trimming the output voltage.
The other great thing about this part is that the output can be set up to provide constant current. An on-board differential amplifier with a level shifted, SGND referenced output is used to sense current on high voltage rails, implementing a current control loop outside the voltage loop. This is ideal for battery charging or for driving LED strings.
There are so many applications for this part in the industrial market that it has quickly become my favourite switching regulator.
What’s your favourite and why?