PROBLEM 077 · Medium · The shunt regulator
Shunt Happens
A shunt regulator for a 1.2 V rail
The problem
Build a shunt regulator. A resistor Rs runs from VDD to the output “out”. An NMOS shunt transistor has its drain on “out”, its source on ground and its gate on the net “gate”. A resistor divider from “out” to ground has its midpoint on “fb”. The test bench provides a 0.6 V reference and an error amplifier that drives “gate” from the difference between “fb” and the reference.
The shunt transistor draws whatever current the load does not need: the current through Rs is constant, so the output holds. Choose Rs so that it carries more than the largest load current plus a small standing current, and choose the divider for an output of 1.2 V.
The test bench
- Supply: VDD = 1.8 V.
- The test bench supplies a 0.6 V reference, an error amplifier (gain 10000) from “fb” to “gate”, a 1 µF output capacitor and a load current from “out” to ground, swept from 0.1 mA to 3 mA.
Numbers to hit
- Output at 1 mA between 1.15 V and 1.25 V
- Drop from 0.15 mA to 3 mA at most 10 mV
- Change per volt of supply at most 40 mV/V
- Every transistor in saturation
Design it yourself
Draw the circuit on a schematic canvas, size every transistor, and see it simulated in your browser with ngspice and the SkyWater SKY130 models. Every transistor lights up green, gold or red by operating region as you move the sliders.
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