Issue
129 April 2001
Have
You Seen the Light?
by
Ed Nisley
Ring
that Bell!
The
MAX629 datasheet recommends single-point grounding for
several components and encourages good circuit board layout.
Figure 4 shows you how important layout can be.
The
square waves leading up to the middle of Figure 4 show
the MAX629 pulling LX to ground and letting the inductor
snap it back to 24 V. The LEDs switch off at the middle
division, where L1 is still charging C2. The MAX629 briefly
yanks LX low, then shuts it off again. Where does the
current flowing through L1 go with D2 biased off and LX
turned off?
Answer:
You’ve just (re)discovered the fact that analog schematics
don’t tell the complete story. Obviously, current
through L1 must flow through either LX or D2. It has nowhere
else to go, right? Well, sorta kinda.
At
RF frequencies, parasitic effects become just as important
as the components you solder to the circuit board. In
this case, current flows through several parasitic capacitors,
across the ground plane, and back to the inductor.
The
resonant frequency of an LC (inductor-capacitor) tank
circuit is, at first approximation:
With
the circuit ringing at about 1.8 MHz and L = 47 mH, the
capacitance is 170 pF. But where’s the capacitor?
For extra credit try to figure out the effective Q of
the LC circuit from the waveform in Figure 4. Hint: Q
is the ratio of stored-to-dissipated energy per cycle.
Bonus: Find the circuit’s effective series resistance
and compare it with L1’s rated DC resistance.
|
(Click
here to enlarge)
|
Figure
4—The voltage at LX rings like a bell when the
load disappears and the MAX629 shuts off the current.
A good ground layout keeps the oscillating current
within the circuit board, not broadcast through space. |
The
largest parasitic capacitance isn’t through the circuit
board to the ground plane. I measured 275 pF of capacitance
on a 16inē circuit board, so the foil on that node is
1 or 2 pF. The inductor’s parasitic capacitance isn’t
more than a few picofarads either.
The
1N5819 datasheet specifies a reverse biased junction capacitance
of 50 to 80 pF. The MAX629 datasheet doesn’t specify
the LX pin’s capacitance, but discrete FETs rated
at 1 A run about 100 pF.
There
you have it. Most of the parasitic capacitance hides inside
the active devices. In a sense, the current does flow
through the LX pin and D2, but not in the way you expect
from the schematic, which illustrates normal circuit operations.
This is why attention to proper grounding and component
layout are important; that current must go somewhere.
Use ugly construction when appropriate, but don’t
let ease of construction outweigh good design.
Once
again, you have been warned.
