There was a window of time in the 1950s and early ’60s when negative voltage rails were commonplace, when germanium PNP transistors were prevalent—like those used in an old “transistor radio”, now worth a fortune on Ebay. Nowadays, NPN transistors are more prevalent, since they basically work better than PNPs. The need for negative voltage rails has rarefied, leaving some circuit designers with little experience facing negative rails.
The modern day usage for negative rails is limited mainly to –48V “telecom” supplies, or as a companion to a positive supply, as for dual op amp rails.
To complicate matters, modern applications sometimes require a variable negative rail. The design presented here uses an LTC3630 synchronous buck converter to convert a +24V input to a variable 0 to –20V output. It’s good for somewhat over 200mA output current at –20V, i.e., 4W.
Please refer to the accompanying LTspice schematics. The LTC3630 is used in “inverting buck” mode, which in the old days was known as “buck-boost” (before our positive-to-positive 4-switch synchronous buck boost converters were available).
Actually, hooked up this way, it’s running in “flyback” mode, characterized by chopped, high di/dt input and output current waveforms. Low-ESR ceramic caps are required at both the input and output, to circulate the AC currents locally (thereby keeping them out of the input and output wiring). Another thing to keep in mind is that the peak switch voltage is equal to VIN + |VOUT|. Also, the power switch and inductor need to carry the sum of both the input and output current waveforms, peaks and all.
Two possible voltage control schemes are shown in Figures 1 and 2. The solution in Figure 1 uses an LT6016 dual op amp. The first amp inverts and gains up a 0V to 5V control signal (presumably from an LTC2630-H DAC) to produce a 0V to –20V control signal. The second amp is used to force VOUT to match the control signal. This design requires a separate, negative supply for the op amps (beyond the main, variable negative output).
Figure 2 shows a simpler way to control the negative VOUT. A single LT6015 op amp attached to the top end of the feedback voltage divider just “pulls” VOUT up and down—no extra power rail required.
A variable voltage rail is somewhat of an unusual requirement, especially when it’s negative, but it does come up from time to time. Generally speaking, it is used to control the power level of something like an ultrasound transducer, or an RF amp. A variable rail can easily be accomplished with most any switching power supply, inverting or not, bucking or boosting. The main point is to use a DAC, with fixed resistors. Using a digital potentiometer as a variable feedback resistor has the unfortunate effect of changing the feedback loop gain, depending on the VOUT setting. Using a DAC sidesteps this issue.