Cheap LDO (or quasi LDO) that will handle ceramic output caps?

[Speakerguy]

Hi,

I need a cheap regulator for 5 to 3.3V regulation. The TLV1117 / LM1117 looks like it will work but it has those pesky requirements for output cap ESR (0R2 < x < 10R). Right now I am planning on a KG series Panasonic that is sort of in the middle of the range (100uF, 2R0 max ESR @ 100KHz).

Is there anything in the sub $1USD range that will work with ceramic output caps (X7R)? Thanks!

Mark

 

[Speakerguy]

Oh, and requirements: Max dropout 1.5V >300ma required (500ma preferable) (SOT-223 preferable).

 

[Hero999]

0R2 to 10R is quite high, why can't you just use a cheap electrolytic with a 1R series resistor?

 

[Speakerguy]

A high ESR electrolytic is cheaper than a good one plus a resistor. The 100UF 2R0 ESR Panny KG's are 11.2 cents each and they are very compact. I'm more worried about my 100nF ceramic IC bypass caps on that are downstream affecting the regulator stability.

 

[Hero999]

I see your point, there's no point in buying an expensive capacitor and ruining it with a resistor.

Adding a series inductor or even a ferrite bead might help.

 

[Speakerguy]

I will put a spot for an 0805 footprint in series after the regulator(s) on the first board I use them on, so I can stick an 0R2 resistor or a FB in place or just bridge it with a clipped resistor lead to evaluate stability. I'm hoping having the ceramics far enough downstream and of a small enough value (so the resonant impedance null is at a high freq) will do the trick.

Surely people have used them like this before without the added resistor or ferrite bead, yes? It seems to be a popular (generic) chip now. They don't cost any more than a standard 7800 series reg, and they are 1% tolerance at low loads to boot (2% @ full 800ma output current).

 

[Hero999]

Is this going to be mass produced?

If it's just a one off hobby project then go ahead. One regulator might work with a low impedance capacitor but one in 100 might oscilate so if you're going to mass produce it then I at least recommend adding some inductance to increase the high frequency impedance.

 

[Speakerguy]

Hobby projects right now, and I've got a scope to check for any problems.

 

[MrAl]

Hi there,

Geeze, i would just try a standard electrolytic.
I've done this with other regulators and they work just fine. The 100uf
electro probably comes in at the right ESR value anyway. If not,
add 0.5 ohms in series. Your choice sounds just right to me however
(2 ohms). The 10x value of C will help with the stability too so
you are probably good to go.

Test with various loads and input voltages, of course.

I dont know what other specs you are looking at, such as ripple rejection,
transient response, etc.

 

[Speakerguy]

Since the "1117" LDO regulator seems to be a generic part, I went searching for data sheets from various companies. Infineon had the best data for the ESR requirements of the capacitor. The relevant page of the data sheet is attached.

I know generic devices can still vary among manufacturers wrt performance specs, but the datasheet clearly shows the .2 to 10 ohm ESR as a worst-case situation at 125C and no load. As temp decreases and load increases it drops dramatically.

I ordered 20pcs of the TI TLV1117 chip (the cheapest 1117 chip out there at 64c in single unit quantities), ten 5V parts and ten 3.3V parts. I will play around with stability tests and loading and output capacitance to see how they behave. It looks as if you load them with more than 50ma current at any temp, or 10ma at 25C, ceramics could be OK (at least with the infineon part). A simple "VOUT GOOD" LED might suffice for stability.

 

[wealth210]

Hi there,

Geeze, i would just try a standard electrolytic.
I've done this with other regulators and they work just fine. The 100uf
electro probably comes in at the right ESR value anyway. If not,
add 0.5 ohms in series. Your choice sounds just right to me however
(2 ohms). The 10x value of C will help with the stability too so
you are probably good to go.

Test with various loads and input voltages, of course.

I dont know what other specs you are looking at, such as ripple rejection,
transient response, etc.

Hi
How to understand ripple rejection and transient response? I understand the value of the ripple rejection value, the bigger and the better.and it is good for having
quickly transient response,right?
I'm selecting LDO for my circuit,but what key parameter should I pay attention?

 

[Speakerguy]

Right now I am planning on a KG series Panasonic that is sort of in the middle of the range (100uF, 2R0 max ESR @ 100KHz).

I've been down that path. Don't do it. I'm at work right now so I can't help out til my lunch break, but I can find something for you.

 

[Speakerguy]

Digi-Key - MCP1824ST-3302E/DBCT-ND (Microchip Technology - MCP1824ST-3302E/DB)

78 cents, SOT-223, 300mA LDO, stable with 1uF ceramic cap or larger.

 

[Speakerguy]

Holy crap, someone thread necromanced MY OWN THREAD from a year ago, and I ended up replying. Good lord.

 

[microtexan]

Own thread reply

Holy crap, someone thread necromanced MY OWN THREAD from a year ago, and I ended up replying. Good lord.

And just think, it will be here forever!!!