L200/L200C Macromodel

Copyright © 2009 Gary R. Van Sickle

The L200 linear voltage regulator from ST Microelectronics is a pretty sweet part. While it is not Low-Drop Out nor does it have a particularly low quiescent current, it is one of the few monolithic linear regulators with built-in user-adjustable current limiting. Even better, this current limiting feature requires only one additional resistor whose value is easily calculated.

In my never-ending quest to design and build the ultimate vacuum tube power supply, this part sounded like a natural for the heater supplies. For a 12AX7-type tube, I’d just set the voltage to 12.6V (heaters in series), set the current limit to, say, twice the hot heater current (i.e. 300ma), and I’d have a nice soft-start heater supply with one IC and 3 resistors per tube. Things rarely get more straightforward than that!

I was sold. Now to simply simulate it with LTSpice to make sure I hadn’t forgotten anything… and we come to a roadblock. ST doesn’t have a SPICE model available. Must be some new 21st-century marketing technique that I don’t quite understand. A thorough search of the World Wide Internet revealed that nobody else had one either. I was therefore forced to roll my own, which brings us to the main course of this banquet of information: Behold, The Shorted Turn‘s L200 / L200C Macromodel Version 1.0!:

Figure 1: L200 Macromodel

The opamps shown are LTSpice’s default “1-pole” opamp models. Reference voltages VREF and VSC were divined from ST Microelectronics’ datasheet on the part. Transistors were chosen almost at random – the 2N2222 because it was there, and the BD137F pass transistor because it had the closest Vce(max) / maximum current handling capacity combination to the L200 of all the transistors currently in my transistor library. Even less thought was put into resistor selection, since the value of neither is especially critical to the operations of the circuit I wished to simulate.

A simple test circuit (for reference in further discussion) is shown in Figure 2:

Figure 2: L200 Macromodel Test Circuit

A wise man once said, “Don’t trust any version under 2.0″, and if ever there was a place where that maxim applied, it’s here. In its current Version 1.0 form, this macromodel models the following:

• The voltage regulation equation given in the L200 datasheet, specifically: V(OUT) = (1+R2/R1)
  where R1 and R2 are the feedback resistors shown in Figure 2.
• The user-adjustable current limiting equation, also as (sort of) given by the L200 datasheet: I(OUT)_MAX = 0.45/R3
  where R3 is the current sense resistor shown in Figure 2.

…and that’s it. Some of the many limitations of this model include:

• Absolutely no thought has been given to loop stability. It doesn’t oscillate in the circuit of Figure 2, nor in the slightly more complex heater regulator circuits I’ve been simulating which cause the current limiting to kick in, but that’s probably due to sheer luck more than anything else.
• While the user-selectable current limiting is modeled, the built-in current limiting (3.6A max) of the L200 is not modeled in any way. The model’s maximum current will be limited by the Q1 model, which as shown is a BD137F. Short-circuit output current of the macromodel simulates out to about 1.9A, considerably lower than the spec.
• Dropout voltage simulates out at about 1.5V, which is about half a volt better than the typical value listed in the datasheet.
• I haven’t even looked at such things as line or load regulation. They are probably considerably better in this model than in the real component due to the use of idealized opamps.
• I have also not looked at quiescent current, which I’m sure is wildly off.

So, there you have it, a macromodel for the L200 Voltage Regulator that’s better than any other you can’t find on the Internet . Let me know if you find this useful, or if you have any ideas for improving the model’s fidelity.

Until next time, I remain,

Gary R. Van Sickle
President and CEO/Editor/Head Writer
The Shorted Turn