LM317 followers will acknowledge Determine 1 as the standard LM317 fixed present supply topology. It carefully regulates Iout = Vadj/Rs by forcing the OUTPUT pin to be Vadj = 1.25 V constructive relative to the ADJ pin. Thus, Iout = Vadj/Rs to an excellent approximation. Grasp chip chef Bob Pease cooked it as much as be so!
Determine 1 Basic LM317 fixed present supply,
Iout = Vadj/Rs + Iadj ≃ Vadj/Rs = 1.25/Rs.
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In typical apply, Iout >> Iadj, the latter being specified at 50 µA typical, 100 µA max. This simplifies the mathematics by making the Iadj bias present safely ignorable with out letting accuracy take a success. It’s labored nice for 50 years nevertheless it has an apparent draw back. the best way you program Iout is by altering Rs.
Determine 2 reveals a brand new(er) topology with a distinct (extra agile) technique for making Iout programmable.
Determine 2 A novel LM317 topology permits management of amps of Iout with simply milliamps of Ic,
Iout = (Vadj – (Ic – Iadj)Rc)/Rs – Ic + Iadj ≃ (Vadj – (Ic – Iadj)Rc)/Rs.
Usually, Rc > 100Rs, making Determine 2 capable of management as much as 1.5 A of Iout with simply milliamps of Ic. In fact, now it could not be ok to simply ignore Iadj.
Determine 3 reveals the concept fleshed out into a whole PWM managed 15 V, 1 A, grounded-load present supply that features Iadj compensation. Right here’s the way it works.
Determine 3 The 1-A, 15-V, PWM-programmed grounded-load present supply with a novel LM317 topology. The asterisked resistors are 1% or higher and Rs = 1.25 Ω.
The 5-Vpp PWM enter has a frequency (Fpwm) assumed to be 10 kHz or thereabouts. If it doesn’t, scale C1 appropriately with:
C1 = 22µF*10kHz/Fpwm
The ensuing PWM switching of Q2 creates a variable resistance averaged by C1 to Rc(1 + 1/Df) the place Df = the 0 to 1 PWM responsibility issue. Thus a (0 to 2.5v)/2Rc = 3.11 mA Ic present = 2.5v/Rc(1 + 1/Df) flows into Z1’s summing level.
Z1 servos the V1 gate drive of Q1 to carry its supply at an correct 2.5-V reference for the PWM conversion and to stage shift Ic to trace U1’s ADJ pin. Additionally summed with Ic is Iadj bias compensation (2.5v/51k = 50µA) supplied by R1.
The unpleasant stack of six 1N4001’s is required to offer bias for Q1 to work into. I freely admit that it’s not very fairly. Hopefully the novelty of Determine 2 makes up for it!
Observe that accuracy and linearity largely rely solely on the match of the Rc resistors and the precision of the Z1 and U1 inner references. It’s a cheerful coincidence that the two:1 ratio of the TL431’s 2.5-V versus the LM317’s 1.25 V permits the handy use of three an identical Rc resistors.
If Rs = 1.25 Ω, then Iout(max) = 1 A and Iout versus Df is as plotted in Determine 4.
Determine 4 Iout versus Df the place Df (x-axis) is the PWM responsibility issue and Iout (y-axis) is Vadj/1.25 = 1 A full-scale = 1 – 2/(1 + 1/Df).
Df versus Iout is plotted in Determine 5.
Determine 5 Df versus Iout the place Iout (x-axis) is 1 A full-scale and Df (y-axis) = 1/(2/(1 – Iout) – 1).
Observe that U1 may be referred to as upon to dissipate as a lot as:
- 20 W if Rs = 1.25 Ω and Iout(max) = 1 A
- 30 W if Rs = 0.83 Ω and Iout(max) = 1.5A
Ethical of the story: don’t be skimpy on the heatsink! Additionally notice that Rs ought to be rated for a wattage of not less than 1.252/Rs.
Then there’s the consideration of energy up/down transients. When the system is first switched on and C1 is sitting discharged, and the controller could have about 4 to eight milliseconds to initialize the PWM logic to 1.0 earlier than C1 can cost sufficient to permit U1 to return on and begin sourcing present. Don’t neglect this element throughout software program improvement! On power-down, Q3 kicks in when +5 V drops under ~2 V. This saturates Q1 and forces Iout to zero to guard the load in addition to discharging C1 in preparation for the subsequent power-up.
In closing, thanks go (once more) to savvy reader Ashutosh for his suggestion that the Determine 2 topology would possibly deserve a centered DI of its personal, and (likewise once more) to editor Aalyia for the fertile DI atmosphere she has created that makes this sort of teamwork, nicely, workable!
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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