14 July 2011

App Note 4

This app note discusses the LT1010, a power buffer designed by Bob Widlar. Bob Widlar contributes the last page and a half to this app note ("The LT1010 at a Glance") discussing the internal details of the design. Incidentally, App Note 16, in which Widlar further discusses this applications of this buffer, is the app note that breaks Jim Williams' initial streak of writing all of Linear Technology's application notes (Jim wrote all of App Notes 1 through 15).

I love the discrete transistor amplifier designs in this note. Figures 4 and 5 are neat designs (even if there is a low-pass filter in the feedback path of the LT1008 in Figure 4). The motor-speed control loop in Figure 13 makes me happy: no mystery transfer functions in this one (the stability of this control loop can be analytically determined!). Also, I can't help but notice that the high-voltage electrostatic piezoelectric fan used in Figure 14 is another example of "Jim finds the weirdest stuff in his junk pile."

The best circuit is the sample-and-hold design with hold-step compensation in Figure 9, and it is worthy of a grad-school lecture in itself. The charge-dump circuit and the TTL interface are valuable examples. My students always have trouble designing a proper interface to TTL levels (dear students, no, a TTL input is not a 0V and 5V voltage source).

(By the way, there's a schematic error in the last figure on the last page (the LT1010 conceptual schematic).  The OUTPUT pin does not connect to the negative rail.  There should be no connection dot on the output: the output pin is only connected to the right side of R1.)

Best quote (page AN4-2): "With C load increased to a brutal 2 uF, the circuit is still stable, even though the large capacitance requires substantial current from the LT1010." Brutal. I love it.

Finally, I note that there are a lot of National LM101A and LM301A op amps used in this app note (Figures 1, 13, and 14). I suppose that if you're working on a project with Bob Widlar, you use his op amps. Full stop.

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