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Purpose of precision high-current series regulator's resistors and capacitor?

Started by Don Kuenz February 11, 2020
This question was originally posted to piclist.

The TL431 datasheet contains "Figure 29. Precision High-Current Series
Regulator." What's the purpose of the 30 Ω resistor, the 4.7 kΩ 
resistor, and the 0.01 μF capacitor?

Thank you,

-- 
Don Kuenz KB7RPU
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.


On Tuesday, 11 February 2020 21:43:48 UTC-5, Don Kuenz  wrote:
> This question was originally posted to piclist. > > The TL431 datasheet contains "Figure 29. Precision High-Current Series > Regulator." What's the purpose of the 30 Ω resistor, the 4.7 kΩ > resistor, and the 0.01 μF capacitor? >
Hi, Don:- There is no "the" TL431 datasheet, as there are multiple versions from TI and at least 8 manufacturers (and probably a few more in China). But presumably this is the schematic: https://i.imgur.com/AyaTqke.png The 4.7K is to keep the output 2N222 from amplifying the leakage in the other 2N222. The other parts are for stability.The 30 ohm for keeping the Darlington from going all Colpitts on us and the cap to keep the TL431 happy. I'm sure someone will point out if this explanation is lacking or incorrect in some way. ;-) I'm not sure the 4.7K and 30 ohm are actually necessary. Best regards, Spehro Pefhany
speff <spehro@gmail.com> wrote:
> On Tuesday, 11 February 2020 21:43:48 UTC-5, Don Kuenz wrote: >> This question was originally posted to piclist. >> >> The TL431 datasheet contains "Figure 29. Precision High-Current Series >> Regulator." What's the purpose of the 30 &#8486; resistor, the 4.7 k&Omega; >> resistor, and the 0.01 &mu;F capacitor? >> > Hi, Don:- > > There is no "the" TL431 datasheet, as there are multiple versions from TI and
at least 8 manufacturers (and probably a few more in China).
> > But presumably this is the schematic: https://i.imgur.com/AyaTqke.png > > The 4.7K is to keep the output 2N222 from amplifying the leakage in the > other 2N222. > > The other parts are for stability.The 30 ohm for keeping the Darlington > from going all Colpitts on us and the cap to keep the TL431 happy. > > I'm sure someone will point out if this explanation is lacking or > incorrect in some way. ;-) > > I'm not sure the 4.7K and 30 ohm are actually necessary.
It may be worthy of a simulation while we wait for the critics. Thank you, -- Don Kuenz KB7RPU There was a young lady named Bright Whose speed was far faster than light; She set out one day In a relative way And returned on the previous night.
speff <spehro@gmail.com> wrote:

> On Tuesday, 11 February 2020 21:43:48 UTC-5, Don Kuenz wrote: >> This question was originally posted to piclist.
>> The TL431 datasheet contains "Figure 29. Precision High-Current Series >> Regulator." What's the purpose of the 30 &#8486; resistor, the 4.7 k&#2013265934; &#2013266089; >> resistor, and the 0.01 &mu;F capacitor?
> Hi, Don:-
> There is no "the" TL431 datasheet, as there are multiple versions from > TI and at least 8 manufacturers (and probably a few more in China).
> But presumably this is the schematic: https://i.imgur.com/AyaTqke.png
> The 4.7K is to keep the output 2N222 from amplifying the leakage in the > other 2N222.
> The other parts are for stability.The 30 ohm for keeping the Darlington > from going all Colpitts on us and the cap to keep the TL431 happy.
> I'm sure someone will point out if this explanation is lacking or > incorrect in some way. ;-)
> I'm not sure the 4.7K and 30 ohm are actually necessary.
> Best regards, > Spehro Pefhany
The url is http://www.ti.com/lit/ds/symlink/tl431.pdf No, they are not. In fact, the first 2N2222 (emitter follower) is not necessary. The pass transistor can supply only a limited current, perhaps 50 mA at 80C, so a darlington is not needed. The TL431 uses a bandgap reference and is extremely noisy, around 450nV/root Hz at 10 Hz, with a pronounced peak just above 10 KHz. However, Fig 29 can be modified to bring the noise down to 3.3nV/root Hz at 1 Hz, and 0.6nV/root Hz at 10 Hz. Here is a 5V regulator in LTspice: Version 4 SHEET 1 1608 1700 WIRE -464 48 -480 48 WIRE -352 48 -464 48 WIRE 96 48 -352 48 WIRE -480 64 -480 48 WIRE -352 64 -352 48 WIRE 96 112 96 48 WIRE -480 160 -480 144 WIRE -352 160 -352 144 WIRE -320 160 -352 160 WIRE -288 160 -320 160 WIRE -176 160 -208 160 WIRE -96 160 -176 160 WIRE -16 160 -96 160 WIRE 32 160 -16 160 WIRE -176 176 -176 160 WIRE -96 176 -96 160 WIRE -16 176 -16 160 WIRE -176 256 -176 240 WIRE -96 256 -96 240 WIRE -16 256 -16 240 WIRE -352 288 -352 160 WIRE -320 288 -352 288 WIRE -240 288 -256 288 WIRE -352 320 -352 288 WIRE -304 352 -320 352 WIRE -240 352 -240 288 WIRE -240 352 -304 352 WIRE -176 352 -240 352 WIRE -112 352 -176 352 WIRE 96 352 96 208 WIRE 96 352 -32 352 WIRE 144 352 96 352 WIRE 160 352 144 352 WIRE 96 368 96 352 WIRE -176 384 -176 352 WIRE -352 400 -352 384 WIRE 96 464 96 448 WIRE -176 480 -176 464 FLAG -352 400 0 FLAG 96 464 0 FLAG -16 160 Q1B FLAG 144 352 Vout FLAG -304 352 U1B FLAG -464 48 VCC FLAG -176 480 0 FLAG -480 160 0 FLAG -320 160 U1C FLAG -176 256 0 FLAG -16 256 0 FLAG -96 256 0 SYMBOL npn 32 112 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL res 80 352 R0 SYMATTR InstName R1 SYMATTR Value 470 SYMBOL cap -192 176 R0 SYMATTR InstName C1 SYMATTR Value 1000uf SYMATTR SpiceLine Rser=12m Lser=10n Rpar=3e6 Cpar=3e-9 SYMBOL voltage -480 48 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 10 SYMBOL res -336 160 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL res -192 368 R0 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL res -16 336 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 10k SYMBOL res -192 144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 10k SYMBOL TL431AS -352 352 M0 SYMATTR InstName X1 SYMBOL cap -256 272 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 1000uf SYMATTR SpiceLine Rser=12m Lser=10n Rpar=3e6 Cpar=3e-9 SYMBOL cap -112 176 R0 SYMATTR InstName C3 SYMATTR Value 1000uf SYMATTR SpiceLine Rser=12m Lser=10n Rpar=3e6 Cpar=3e-9 SYMBOL cap -32 176 R0 SYMATTR InstName C4 SYMATTR Value 1000uf SYMATTR SpiceLine Rser=12m Lser=10n Rpar=3e6 Cpar=3e-9 TEXT -488 -64 Left 2 ;'TL431 Noise Spectrum TEXT -88 -32 Left 2 !.lib TL431AS.lib TEXT -488 -32 Left 2 !.noise V(vout) V1 oct 1000 1 1e6 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Noise Spectral Density - (V/Hz&#2013266109; or A/Hz&#2013266109;)] { Npanes: 1 { traces: 1 {524290,0,"V(onoise)"} X: ('M',0,1,0,1e+006) Y[0]: ('n',1,5e-010,5e-010,6.5e-009) Y[1]: ('_',0,1e+308,0,-1e+308) Units: "V/Hz&#2013266109;" ('n',0,0,1,5e-010,5e-010,6.5e-009) Log: 1 0 0 GridStyle: 1 } } [AC Analysis] { Npanes: 1 { traces: 1 {524290,0,"V(vout)"} X: ('M',1,0.001,0,1e+007) Y[0]: (' ',0,1.99526231496888e-006,6,0.00398107170553497) Y[1]: (' ',0,80,20,280) Volts: (' ',0,0,5,2.500764,6e-006,2.500842) Amps: ('m',0,0,4,0.01000308,2e-008,0.01000334) Log: 1 2 0 GridStyle: 1 PltMag: 1 } } [Transient Analysis] { Npanes: 1 { traces: 1 {589827,0,"V(vout)"} X: (' ',1,0,0.2,2) Y[0]: (' ',4,4.9938,0.0001,4.9952) Y[1]: ('m',4,1e+308,3e-007,-1e+308) Volts: (' ',0,0,4,4.9938,0.0001,4.9952) Log: 0 0 0 GridStyle: 1 PltMag: 1 PltPhi: 1 0 } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * TL431AS.lib * Models have been edited for LTspice syntax, but are otherwise per originals * Model developed by analogspiceman ************************************************** .subckt TL431AS A K R ; Anode Kathode Reference D1 R K Dc R1 A R 1.3e6 tc1=3m G1 A 5 R 4 1 C1 5 A 1n Rpar=600 D2 7 R Dc D3 7 4 Dn R2 7 4 950k R3 6 5 600k D4 A 6 Di C2 K 6 40p Rser=5k Rpar=1e6 G2 K 7 6 A 95m D5 A 7 2V5 D6 7 A Dk C3 7 A 50p D7 A K Dc .model Dc d Ron=10 Vfwd=0.65 Vrev=36 Epsilon=50m .model Dn d Is=1p Kf=0p2 Cjo=0p3 .model Di d Ron=10m epsilon=1m .model Dk d Ron=5k7 Vfwd=0.8 Epsilon=0.5 .model 2V5 d Ron=27m Vfwd=0.6 Epsilon=10m Vrev={Vr} revEpsilon=10m .param Vr=2.5+dt*(95u-dt*2u6) dt=temp-10 .ends TL431AS
Steve Wilson <no@spam.com> wrote:

> speff <spehro@gmail.com> wrote: > >> On Tuesday, 11 February 2020 21:43:48 UTC-5, Don Kuenz wrote: >>> This question was originally posted to piclist. > >>> The TL431 datasheet contains "Figure 29. Precision High-Current Series >>> Regulator." What's the purpose of the 30 &#8486; resistor, the 4.7 k&#2013265934; &#2013266089; >>> resistor, and the 0.01 &mu;F capacitor? > >> Hi, Don:- > >> There is no "the" TL431 datasheet, as there are multiple versions from >> TI and at least 8 manufacturers (and probably a few more in China). > >> But presumably this is the schematic: https://i.imgur.com/AyaTqke.png > >> The 4.7K is to keep the output 2N222 from amplifying the leakage in the >> other 2N222. > >> The other parts are for stability.The 30 ohm for keeping the Darlington >> from going all Colpitts on us and the cap to keep the TL431 happy. > >> I'm sure someone will point out if this explanation is lacking or >> incorrect in some way. ;-) > >> I'm not sure the 4.7K and 30 ohm are actually necessary. > >> Best regards, >> Spehro Pefhany > > The url is http://www.ti.com/lit/ds/symlink/tl431.pdf > > No, they are not. In fact, the first 2N2222 (emitter follower) is not > necessary. The pass transistor can supply only a limited current, > perhaps 50 mA at 80C, so a darlington is not needed. > > The TL431 uses a bandgap reference and is extremely noisy, around > 450nV/root Hz at 10 Hz, with a pronounced peak just above 10 KHz. > > However, Fig 29 can be modified to bring the noise down to 3.3nV/root Hz > at 1 Hz, and 0.6nV/root Hz at 10 Hz. Here is a 5V regulator in LTspice:
Forgot the ASY and some other files. That's too many to handle, so here's the whole thing zipped: https://drive.google.com/open?id=162h1_AQ-rCfFou8LaHrKZOpMlGP1PgJP This uses Helmut's model, and produces 2 nV/root Hz at 1 cycle.
On 12/02/2020 21:24, Steve Wilson wrote:
> Steve Wilson <no@spam.com> wrote: > >> speff <spehro@gmail.com> wrote: >> >>> On Tuesday, 11 February 2020 21:43:48 UTC-5, Don Kuenz wrote: >>>> This question was originally posted to piclist. >> >>>> The TL431 datasheet contains "Figure 29. Precision High-Current Series >>>> Regulator." What's the purpose of the 30 &acirc;&bdquo;&brvbar; resistor, the 4.7 k&Icirc; &copy; >>>> resistor, and the 0.01 &Icirc;&frac14;F capacitor? >> >>> Hi, Don:- >> >>> There is no "the" TL431 datasheet, as there are multiple versions from >>> TI and at least 8 manufacturers (and probably a few more in China). >> >>> But presumably this is the schematic: https://i.imgur.com/AyaTqke.png >> >>> The 4.7K is to keep the output 2N222 from amplifying the leakage in the >>> other 2N222. >> >>> The other parts are for stability.The 30 ohm for keeping the Darlington >>> from going all Colpitts on us and the cap to keep the TL431 happy. >> >>> I'm sure someone will point out if this explanation is lacking or >>> incorrect in some way. ;-) >> >>> I'm not sure the 4.7K and 30 ohm are actually necessary. >> >>> Best regards, >>> Spehro Pefhany >> >> The url is http://www.ti.com/lit/ds/symlink/tl431.pdf >> >> No, they are not. In fact, the first 2N2222 (emitter follower) is not >> necessary. The pass transistor can supply only a limited current, >> perhaps 50 mA at 80C, so a darlington is not needed. >> >> The TL431 uses a bandgap reference and is extremely noisy, around >> 450nV/root Hz at 10 Hz, with a pronounced peak just above 10 KHz. >> >> However, Fig 29 can be modified to bring the noise down to 3.3nV/root Hz >> at 1 Hz, and 0.6nV/root Hz at 10 Hz. Here is a 5V regulator in LTspice: > > Forgot the ASY and some other files. That's too many to handle, so here's > the whole thing zipped: > > https://drive.google.com/open?id=162h1_AQ-rCfFou8LaHrKZOpMlGP1PgJP > > This uses Helmut's model, and produces 2 nV/root Hz at 1 cycle. >
Hm. I think you might not really get that performance if you build it. Base spreading resistance might be a problem and you might need at least some special transistors to fix that.
Chris Jones wrote...
> >On 12/02/2020 21:24, Steve Wilson wrote: >> Steve Wilson <no@spam.com> wrote: >> >>> speff <spehro@gmail.com> wrote: >>> >>>> On Tuesday, 11 February 2020 21:43:48 UTC-5, Don Kuenz wrote: >>>>> This question was originally posted to piclist. >>> >>>>> The TL431 datasheet contains "Figure 29. Precision High-Current Series >>>>> Regulator." What's the purpose of the 30 &acirc;&bdquo;&brvbar; resistor, the 4.7 k&Icirc; &copy; >>>>> resistor, and the 0.01 &Icirc;&frac14;F capacitor? >>> >>>> Hi, Don:- >>> >>>> There is no "the" TL431 datasheet, as there are multiple versions from >>>> TI and at least 8 manufacturers (and probably a few more in China). >>> >>>> But presumably this is the schematic: https://i.imgur.com/AyaTqke.png >>> >>>> The 4.7K is to keep the output 2N222 from amplifying the leakage in the >>>> other 2N222. >>> >>>> The other parts are for stability.The 30 ohm for keeping the Darlington >>>> from going all Colpitts on us and the cap to keep the TL431 happy. >>> >>>> I'm sure someone will point out if this explanation is lacking or >>>> incorrect in some way. ;-) >>> >>>> I'm not sure the 4.7K and 30 ohm are actually necessary. >>> >>>> Best regards, >>>> Spehro Pefhany >>> >>> The url is http://www.ti.com/lit/ds/symlink/tl431.pdf >>> >>> No, they are not. In fact, the first 2N2222 (emitter follower) is not >>> necessary. The pass transistor can supply only a limited current, >>> perhaps 50 mA at 80C, so a darlington is not needed. >>> >>> The TL431 uses a bandgap reference and is extremely noisy, around >>> 450nV/root Hz at 10 Hz, with a pronounced peak just above 10 KHz. >>> >>> However, Fig 29 can be modified to bring the noise down to 3.3nV/root Hz >>> at 1 Hz, and 0.6nV/root Hz at 10 Hz. Here is a 5V regulator in LTspice: >> >> Forgot the ASY and some other files. That's too many to handle, so here's >> the whole thing zipped: >> >> https://drive.google.com/open?id=162h1_AQ-rCfFou8LaHrKZOpMlGP1PgJP >> >> This uses Helmut's model, and produces 2 nV/root Hz at 1 cycle. > > Hm. I think you might not really get that performance if you build it. > Base spreading resistance might be a problem and you might need at > least some special transistors to fix that.
Yep, much better parts in Table 8.1 in AoE3. Also, the circuit model needs to be stable with an output capacitor.
>
-- Thanks, - Win
Winfield Hill <winfieldhill@yahoo.com> wrote:

> Chris Jones wrote... >> >>On 12/02/2020 21:24, Steve Wilson wrote: >>> Steve Wilson <no@spam.com> wrote:
>>>> The url is http://www.ti.com/lit/ds/symlink/tl431.pdf
>>>> No, they are not. In fact, the first 2N2222 (emitter follower) is not >>>> necessary. The pass transistor can supply only a limited current, >>>> perhaps 50 mA at 80C, so a darlington is not needed.
>>>> The TL431 uses a bandgap reference and is extremely noisy, around >>>> 450nV/root Hz at 10 Hz, with a pronounced peak just above 10 KHz.
>>>> However, Fig 29 can be modified to bring the noise down to 3.3nV/root >>>> Hz at 1 Hz, and 0.6nV/root Hz at 10 Hz. Here is a 5V regulator in >>>> LTspice:
>>> Forgot the ASY and some other files. That's too many to handle, so >>> here's the whole thing zipped:
>>> https://drive.google.com/open?id=162h1_AQ-rCfFou8LaHrKZOpMlGP1PgJP
>>> This uses Helmut's model, and produces 2 nV/root Hz at 1 cycle.
>> Hm. I think you might not really get that performance if you build it. >> Base spreading resistance might be a problem and you might need at >> least some special transistors to fix that.
Adding 50 ohms in series with the Q1 base increases the noise to 2.2 nV/Hz^ 1/2. Base spreading resistance is not a problem. The 2N3904 is a surprisingly good transistor. It also has an entry in both LTspice IV and XVII
> Yep, much better parts in Table 8.1 in AoE3.
With Spice Models? What's wrong with 0.6 nV/Hz^1/2 at 10 Hz?
> Also, the circuit model needs to be stable with an output capacitor.
Adding a 1000uF from Vout to gnd drops the noise from 0.6 nV/Hz^1/2 to 0.02 nV/Hz^1/2 past 10 KHz. The capacitors have 12 milliohms series resistance and 10 nH series inductance. The transient response shows 209uV overshoot with a 0.1V step at the input. It damps out in 2 cycles. The output voltage settles 80uV higher.
On Tuesday, February 11, 2020 at 10:20:36 PM UTC-5, speff wrote:
> > There is no "the" TL431 datasheet, as there are multiple versions from TI and at least 8 manufacturers (and probably a few more in China). > > But presumably this is the schematic: https://i.imgur.com/AyaTqke.png > > The 4.7K is to keep the output 2N222 from amplifying the leakage in the > other 2N222.
Wasn't the 2N222 obsolete in 1970? :)
Michael Terrell <terrell.michael.a@gmail.com> wrote:
> On Tuesday, February 11, 2020 at 10:20:36 PM UTC-5, speff wrote: >> >> There is no "the" TL431 datasheet, as there are multiple versions from >> TI and at least 8 manufacturers (and probably a few more in China). >> >> But presumably this is the schematic: https://i.imgur.com/AyaTqke.png >> >> The 4.7K is to keep the output 2N222 from amplifying the leakage in the >> other 2N222. > > > Wasn't the 2N222 obsolete in 1970? :)
My 1974 Archer _Transistor Substitution Guide_ classifies the 2N222 as a PNP low power alloy-junction germanium transistor, which doesn't jibe with the NPN transistors shown in the schematic. Piclist's inimical Bob Blick says: RB pinches off the base of the final transistor and makes the Darlington configuration faster and more linear. RA is called a base stopper and destroys the Q of what could otherwise be a resonant circuit due to combination of the output load on the regulator, interelectrode capacitances in the transistors, and the output gain. Emitter followers are quite susceptible to oscillation. C1 adds local feedback to the TL431 to reduce its gain long before the phase shift of the output transistors can cause a positive feedback situation. Basically, those three components all are there to give stability. Depending on the type of load, they may not be needed. The TL431 by itself is a pretty slow device, but the writers of the app note probably wanted to protect against a worst-case scenario. BTW, there are lots of TL431 variations from different manufacturers. Just because it's marked TL431 doesn't mean it is anything like one from TI. Including the pinout :) Thank you, -- Don Kuenz KB7RPU There was a young lady named Bright Whose speed was far faster than light; She set out one day In a relative way And returned on the previous night.