The venerable sawtooth generator, also known as the ramp generator, analog
time base, etc., is one of the standard building blocks in the linear designer's
bag of tricks. Many implementations of this old standby exist, but most consist
of an integrator, ramp reset switch, and a pulse generator to control the reset
switch.
Of those, the reset pulse generator usually represents the messiest part of
the design of a sawtooth generator. The lower limit of the reset pulse's duration
is set by the minimum time needed to reset the integrator. At the upper end,
it's limited to the longest "front porch" at the onset of the ramp
that can be tolerated by the designer who's expecting a nice, sharp vertex.
Usually these criteria diverge and dictate a compromise that's less than an
elegant solution to either requirement.
With the circuit shown, the reset pulse issue can be avoided altogether by
using a "charge pump" technique instead of the integrator reset switch
(see the figure). S1, a CMOS SPDT switch,
accepts the digital timing control signal. When the control signal is low, S1
connects the charge-pump capacitor (C2) to the output of the integrator.
Thus, as the integrator ramps toward the peak output voltage (Vp),
C2 tracks the ramp and will hold a charge of Vp)C2
when the control signal switches. This charge will then be transferred by S1
to the integrator input, causing a reset step of - Vp)C2/C1.
If the duty cycle of the time control signal is anywhere near 50%, so that adequate
times are allowed for the charge/discharge of C2 through R2
plus the on-resistance of S1, then proper operation is assured and no special
conditioning of the timing signal is necessary.
Once started, the converter will converge rapidly over a few cycles of operation
to a steady state where the reset step Vp)C2/C1
equals the ramp amplitude: Ts (Vcm– V1)/R1
C1. This convergence will be successful provided that the condition
C2< 2C1 is satisfied.
If, for example, C2 = C1, then Vp will equal
the ramp amplitude. That will cause reset to drive the sawtooth voltage to Vcm.
Much of the versatility of this converter, however, results from what happens
if C2 is made different from C1. The reset voltage of
the sawtooth is given by Vr = Vp (1 – C2/C1).
So, by properly selecting the C2/C1 ratio, the sawtooth's
baseline can be moved about easily over a wide range. In the example circuit,
this feature is used to generate a rail-to-rail ramp, which is usually hard
to do with a single amplifier.
Note that R2 should be chosen to limit the reset current peak to
a reasonable value, and that Vcm– Vi input voltages
of either polarity work equally well.
