There are a number of technique of producing analog sawtooth waveforms. Right here’s a technique that employs a single provide voltage rail and isn’t finnicky about passive part values. Determine 1 exhibits a pair of circuits that use a single 3.3-V provide rail, one producing a ground-referenced sawtooth and the opposite a provide voltage-referenced one.
Determine 1 The circuitry to the left of the three.3 V provide implements a ground-referenced sawtooth labeled “LO”, whereas that to the best types a 3.3V-referenced one labeled “HI”.
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For the LO sign, R1 provides satisfactory present to function U1. This IC enforces a relentless voltage Vref between its V+ and FB pins. Q1 is a excessive beta NPN transistor which passes just about all of R2’s present (Vref/R2) via its collector to cost C1 with a relentless present, producing the linear ramp portion of this ground-referenced sawtooth. (U1’s FB present is usually lower than 100 nA over temperature.) M1 is a MOSFET that’s activated for 100 ns each T seconds to quickly discharge C1 to floor. Its “on” resistance is lower than 1 Ω and so yields a discharge that lasts greater than 10 time constants.
The sawtooth’s peak amplitude A is Vref × T / (R2 × C1) volts, the place Vref for U1 is 1.225 V. For a 3.3-V rail, the amplitude (A) must be lower than an Amax of two.1 V, which requires T to be lower than a Tmax of R2 × C1 × 2.1V / Vref. With the supply of a U1 Vref tolerance of 0.2% and a 0.1% tolerance for R2, the circuit’s total amplitude tolerance is generally restricted by an at finest 1% C1 mixed with the parasitic capacitance of M1.
M2, C2, Q2, R3, R4 and U2 work very similar to the circuit simply described, besides that they produce an “upside-down” 3.3-V supply-referenced sawtooth. Each waveforms might be seen in Determine 2. Except U2, the tolerance contributions of those elements are these beforehand talked about for the “proper side-up” design respectively. U2’s reference present is usually lower than 250 nA over temperature, however its Vref of 1.24 V has at finest a 1% tolerance. Determine 2 depicts each sawtooth waveforms.
Determine 2 The waveforms proven have peak values that are barely lower than the most important really useful. The interval T is 34 µs.
These circuits don’t require any precision or matched-value passive elements. And there’s no must coordinate these part values with any energetic part’s parametric values or with the switching interval T, so long as T is stored lower than Tmax. The one impact that the non-zero tolerances of the passive elements and of sure energetic parameters has been on the peak-to-peak amplitude of the sawtooth waveforms.
Christopher Paul has labored in numerous engineering positions within the communications business for over 40 years.
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