This design thought revisits one other: “PWM energy DAC incorporates an LM317.” Like the sooner circuit, this one implements an influence DAC by integrating an LM317 constructive regulator right into a largely passive PWM topology. It exploits the built-in options of that time-proven Bob Pease masterpiece in order that its output is proportional to the assured 2% precision of the LM317 inner voltage reference and is inherently protected against overloading and overheating.
Wow the engineering world along with your distinctive design: Design Concepts Submission Information
Nevertheless, in contrast to the sooner design concept that requires a separate 15v DC energy enter, this remake (proven in Determine 1) provides a switching enter increase preregulator so it could actually run from a 5v logic rail. The earlier linear design additionally has a restricted energy effectivity that really drops under single-digit percentages when driving low voltage masses. The preregulator fixes that by monitoring the input-output voltage differential throughout the LM317 and maintains a continuing 3v. That is the simply sufficient dropout-suppressing headroom for the LM317, minimizing wasted energy.
Right here’s the way it works.
Determine 1 LM317 and HC4053 mix to make a PWM energy DAC whereas Q1 forces preregulator U3 to trace and preserve a continuing 3v U2 I/O headroom differential to enhance effectivity.
As described within the earlier DI, switches U1b and U1c settle for a 10-kHz PWM sign to generate a 0v to 11.25v “ADJ” management sign for the U2 regulator by way of suggestions networks R1, R2, and R3. The incoming PWM sign is AC coupled in order that U1 can “float” on U2’s output. U1c supplies a balanced inverse of the PWM sign, implementing lively ripple cancellation as described in “Cancel PWM DAC ripple with analog subtraction.”
Observe that R1||R2 = R3 to optimize ripple subtraction and DAC accuracy. This suggestions association makes U2’s output voltage comply with this operate of PWM responsibility issue (DF):
Vout = 1.25 / (1 – DF(1 – R1/(R1 + R2))) = 1.25 / (1 – 0.9 DF),
as graphed in Determine 2.
Determine 2 Vout (1.25v to 12.5v) versus PWM DF (0 to 1) the place Vout = 1.25 / (1 – 0.9 DF).
Determine 3 plots the inverse of Determine 2, yielding the PWM DF required for any given Vout.
Determine 3 The inverse of Determine 2 or, the PWM DF required for any given Vout, the place PWM DF = (1.111 – 1.389/Vout).
About that monitoring preregulator factor: Management of U3 to keep up the 3v of headroom required to carry U2 secure from dropout depends on Q1 appearing as a easy (however sufficient) differential amplifier. Q1 drives U3’s Vfb voltage suggestions pin to keep up Vfb = 1.245v. Subsequently (the place Vbe = Q1’s emitter-base bias):
Vfb/R7 = ((U2in – U2out) – Vbe)/R6
1.245v = (U2in – U2out – 0.6v)/(5100/2700)
U2in – U2out = 1.89 * 1.245v + 0.6v = 3v
In the meantime, deducing what Q2 does is left as an train for the astute reader. Trace: It saves a few third of a wattage over the unique DI at Vout = 12v.
Observe, if you wish to use this circuit with a special preregulator with a special Vfb, simply modify:
R7 = R6 Vfb/2.4
In closing…
Thanks should go to reader Ashutosh for his intelligent suggestion to enhance energy DAC effectivity with a monitoring regulator, additionally (and particularly) to editor Aalyia for her creation of a Design Concept atmosphere that encourages such free and pleasant cooperation!
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
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- Parsing PWM (DAC) efficiency: Half 1—Mitigating errors
- Cancel PWM DAC ripple with analog subtraction however no inverter
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