Connecting a Voltage Regulators - TL750L05CLP

Hi guys,

Thanks for the help. After I posted this message, I added a 3.3 uF capacitor between the output and the ground and this seems to have stablized the output to 4.9 volts. I would really like to have a 5 volt output so I tried a .47 uF cap and the output seems to change as you put more load on it (not good). So, I looked for a 1 uF cap but couldnt find one. Would using a 1 uF cap bring my output voltage to 5 volts?

The input can't be higher than 6 volts (4 AAA batteries) so do you think I should switch to a different regulator? someone mentioned LM2936 regulator, what do you think? worth the trouble of changing regulators?

I also had a .01uF cap from the input to the ground but this seems to make no difference in the output voltage, should it?

Thanks, Aria

hi again,

here's the new schematic

if anyone's interested.

Your connection is correct, although you may need to add an input bypass capacitor, or - and an output electrolytic capacitor depending upon your circuit.

6 volts is not much "overhead" (1 volt!) for this voltage regulator. That is, the regulator needs a few volts above the regulated voltage to maintain regulation. Although the datasheet does say that a minimum of 6V is required , I suspect that when you place your load onto the 5V reg output, the input is going somewhat lower than 6V. How much current does your load (device) require? Can you use a higher voltage as the input to the regulator? If you describe in some detail your project, we can better advise you.

Thanks Meindert. Just for my future reference, how do you know? does it say that somewhere in the datasheet?

You say the output seems to go up and down as the input changes. The lowest input voltage you can use is 6 volts. The maximum current you can draw from the output is 150 mA. So, what is the load resistance? It *must* be greater than

33 ohms. (I'd recommend using a 100 ohm resistor.) And, when you talk about the input changing, do you have specifics? Finally, you need a 10uf tantalum capacitor from output to ground to match the scant information on the datasheet. You could probably use a low ESR aluminum electrolytic instead.

Ed

Ooops - meant to mention one more thing: you must have a load on the regulator. If you are measuring v out without a load you won't get decent regulation.

Ed

Low drop out regulators are especially picky about the output capacitor. Take a look at figure 4 on the data sheet. It shows how much resistance must be *in series with* the output capacitor to keep the regulator stable versus load current, when using a 10 uF capacitor. The acceptable series resistance for 15 mA to 150 MA (note the bottom scale change from 5 mA per division to 40 mA per division, indicated by bold versus thin vertical lines) must fall in a 16 to one range of .024 to .4 ohms. Most people don't even know that capacitors have internal resistance. For load current below 15 ma, the resistance can be higher. Other values of output capacitance would have a different set of limits for series resistance, and since they don't give you the other curves, most people will just use 10 uF. Production designs usually require a full temperature check and guarantees on capacitor internal resistance if the thing must work.

If doing a through hole design, I usually use a stacked film capacitor (that has a very low internal resistance) and add a resistor in series, so I know what the situation is. Quite often, a 1uF stacked film in series with something like 0.22 ohms and that paralleled by an aluminum electrolytic of 10 to 22 uF will work fine, at least at room temperature.

If doing an SMT design, I might use a 10 uF ceramic like Digikey # PCC2326C 10 uF, 25 V 1206 size X5R type low series resistance capacitor (a couple milliohms), with a series 0.22 or 0.33 ohm resistor.

Either set needs to be very close to the regulator output and common pins.

You need to define what output voltage is acceptable, under which load. No regulator part will be specified to provide 5.0V *exactly*. The precision of the regulation is specified in the datasheet and need to be compatible with your requirements.

In the normal operation, the bigger the value of the capacitor the less the output voltage will vary with the changing input voltage and the load. But this will not shift much the average value - which you are probably measuring.

This is not a very efficient use of the batteries' capacity - much of their capacity will still be there when their total output voltage will drop to 5V+ voltage_regulator_dropout or below. And you won't have enough voltage at all if the accumulators are used.

You should either increase the number of batteries, or use a switching regulator which can upconvert - but the latter option would probably be too much a challenge given your current knowledge of circuit design.

-- Andy

This regulator needs a 10uF decoupling capacitor at the output to be stable.

Meindert

You need a completley different chip and circuit if the input source you have must be 4 AAA cells. Something like a TL499 would give you good regulation at 5V output with a 100 mA draw as the batteries faded down from 6V to below 3 volts. If current consumption is lower, it can provide 5 volts as the batteries fade even further. The maximum current it can provide is 100 mA. If your load is under 100 mA, this, or an equivalent, is the chip and ciruit to use.

See the datasheet at:

Ed

From personal experience. I designed a battery operated device, which ran an

80c51 micro from a 9V battery and used a TL750L05 to get 5V for the micro. I first had 1u before and after the regulator and saw the 5V really being a sawtooth-like voltage, oscillating between 2 and 6V. I then replaced the output capacitor with a 10u one, which solved the problem.

But as other have pointed out, running this from 4 cells at 6V won't give you much operating time, since 6V is the minimum input voltage. There are better linear regulators available which work with only 150mV voltage drop.

Meindert