Ultra-Low Dropout Regulator

MikeMl · Dec 28, 2009

I have a need for an Ultra-Low Dropout Voltage Regulator which will deliver ~1/2A at 12V from an automotive battery/alternator system when the battery voltage ranges from 12V (engine stopped) to 14.5V (engine spooled up and battery fully charged).

Anybody have anything better than this?

Vizier87 · Dec 29, 2009

I'm sorry if it doesn't help, Mike, but I thought those ubiquitous LM317 adjustable regulators give an adjustable 1.2-25V at 1.5A?

Again, I'm sorry if this looks like a newbie nose sticking in.

MikeMl · Dec 29, 2009

Vizier87 said:
...I thought those ubiquitous LM317 adjustable regulators give an adjustable 1.2-25V at 1.5A?...

Yes they do, but their Drop-Out voltage is 1.6 to 2.5V, which means that to make 12.0V out, you have to feed in >~14V. I'm trying to get 12.00V out with only 12.01V in

Roff · Dec 29, 2009

You might want to check transient response. I would worry about oscillation, which won't show up on a supply voltage sweep.

crutschow · Dec 29, 2009

Perhaps this TI device **broken link removed** will work for you. It has about a 200mV dropout at 500mA.

MikeMl · Dec 29, 2009

Transient response is not too bad. Top one is sudden onset of load; bottom one is load-dump.

Roff · Dec 29, 2009

MikeMl said:
Transient response is not too bad. Top one is sudden onset of load; bottom one is load-dump.

Looks pretty good. Would you mind posting the .ASC file?

MikeMl · Dec 29, 2009

Roff said:
...Would you mind posting the .ASC file?

See above.

audioguru · Dec 29, 2009

Where is the model of the LM431?

MikeMl · Dec 29, 2009

audioguru said:
Where is the model of the LM431?

On my hard drive

Roff · Dec 29, 2009

Mike, it oscillates in my sim when the load is connected. I'm guessing my model must be different than yours.
Here is the model I have:

Code:

.subckt TL431 REFIN ANODE CATHODE 
Q8 Q2_C Q2_C R4_N 0 P1 
Q9 CATHODE Q7_C Q9_E 0 n1 
R8 Q4_E ANODE 800 
R9 Q2_E R9_N 4k 
Q10 CATHODE R6_P ANODE 0 n1 5 
Q11 Q7_C R10_P ANODE 0 n1 
*.nodeset R3_N 1 
C2 R9_N C2_N 20p 
R10 R10_P R1_N 1k 
C1 CATHODE Q7_C 20p 
D1 ANODE Q7_C DIODE 
D2 ANODE CATHODE DIODE 
R4 CATHODE R4_N 800 
Q2 Q2_C Q1_E Q2_E 0 n1 
Q3 R9_N C2_N ANODE 0 n1 
R5 CATHODE R5_N 800 
R6 R6_P Q9_E 150 
Q1 CATHODE REFIN Q1_E 0 n1 
R7 ANODE R6_P 10k 
Q6 Q7_C Q7_C REFIN 0 n1 
Q7 Q7_C Q2_C R5_N 0 P1 
R1 R3_N R1_N 2.4k 
R2 R3_N C2_N 7.2k 
Q4 C2_N R1_N Q4_E 0 n1 
Q5 R1_N R1_N ANODE 0 n1 
R3 Q1_E R3_N 3.28k 
.model p1 pnp bf=50 
.model n1 npn bf=100 tf=2n cjc=1p is=5e-18 NF=1.07 
.model diode d rs=1 cjo=2p 
.ends

EDIT: I posted this before I saw that you had posted your model. I'll take a look at it.

Roff · Dec 29, 2009

Your TL431 model appears to have no poles. That may make the difference between oscillating and not oscillating.

MikeMl · Dec 29, 2009

What is the source of your model?

Roff · Dec 29, 2009

MikeMl said:
What is the source of your model?

I have no idea. On my computer, it is dated 12/12/05. I looked online at Fairchild, TI, and Onsemi models, and it was none of those.

Mr RB · Dec 29, 2009

For a low current app like 0.5 amp I would just use a 5v low dropout low Q current regulator, and 2 resistors to make it output 12v (the same system you use on a LM317).

Just think of it as a low dropout LM317 that has Vadj at 5v not 1.2v.

Roff · Dec 30, 2009

Mr RB said:
For a low current app like 0.5 amp I would just use a 5v low dropout low Q current regulator, and 2 resistors to make it output 12v (the same system you use on a LM317).

Just think of it as a low dropout LM317 that has Vadj at 5v not 1.2v.

So would I. Please provide a part number.

MikeMl · Dec 30, 2009

Mr RB said:
...I would just use a "5v low dropout low Q current regulator"...

Do you have a specific part in mind? The Vdo for a e.g. LM7805 is too high...

Sceadwian · Dec 30, 2009

What you need is a SEPIC DC-DC converter. I doubt you'll ever find a linear regulator that can do what you're asking, unless it has charge pumps secondary regulators and overly complicated circuitry.

Roff · Dec 30, 2009

The transient response of this looks pretty good. Too bad CA3096 doesn't seem to be readily available. The transistors would match better, and have better thermal coupling.

Blueteeth · Dec 30, 2009

There are many 1-3A LDO regulators which have excellt transient response and can cope with an automotive enviroment. I recently got a load of UCC283-ADJ form ebay. around 200mV dropout for 1.5A out.

If you need absolute dropout, then I'm afraid a switching converter is the only way to go. Perhaps a sepic, or even a boost converter (with minimal voltage gain) followed by a fast transient LDO linear regulator. Perhaps using a P-MOSFET to switch the output to the input below a certain threshold (some LDO's do this), that way, below 12.2V you can use the resistance of the MOSFET to drop this down to 12v.

Just a few idea's.

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