This design concept 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 principally 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
Nonetheless, 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 possibly run from a 5v logic rail. The earlier linear design additionally has a restricted energy effectivity that really drops beneath single-digit percentages when driving low voltage hundreds. The preregulator fixes that by monitoring the input-output voltage differential throughout the LM317 and maintains a continuing 3v. That is the simply satisfactory dropout-suppressing headroom for the LM317, minimizing wasted energy.
Right here’s the way it works.
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 through suggestions networks R1, R2, and R3. The incoming PWM sign is AC coupled in order that U1 can “float” on U2’s output. U1c offers a balanced inverse of the PWM sign, implementing energetic ripple cancellation as described in “Cancel PWM DAC ripple with analog subtraction.”
Be aware 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 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 take care of the 3v of headroom required to carry U2 protected from dropout depends on Q1 appearing as a easy (however satisfactory) differential amplifier. Q1 drives U3’s Vfb voltage suggestions pin to take care of 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.
Be aware, if you wish to use this circuit with a unique preregulator with a unique Vfb, simply regulate:
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 Thought atmosphere that encourages such free and pleasant cooperation!
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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