LDO precision--L78L33 voltage regulator to 3.3V

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Hi,

I am using the L78L33 (http://www.st.com/stonline/books... ) to produce a 3.3V output, wired up as shown in the attached image. My Vin = 5V basically from my computer's USB port.

Here is what I observed:

1. Cin = 0.1uF or Cin = 0.30uF (since I don't have a 0.33uF) cap and Cout = 0.1uF, I get Vout = 3.78V. This is on the breadboard.

I think I can solve this to Vout = 3.38V with a rectifier diode of 0.40V drop in series with the Vout.

(Same results if I used ceramic or multi-layered ceramic caps.)

2. Here is what puzzles me (and without a rectifier diode) I actually created a breakout board with both Cin = Cout = 0.1uF using the multi-layered ceramic caps. I get consistently 4.33V--very stable. Again my Vin is 5V but with a polarized 100uF of 16V in parallel to the Vin.

Why such a big difference between cases 1 and 2? Thanks for any tips.

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Last Edited: Mon. Nov 15, 2010 - 05:24 AM
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The regulator is not wired up correctly or your meter is faulty.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Or you don't draw any current from it so output is unstable.

Or your capacitors are too far from the regulator, and the output oscillates.

Anyway the breadboard is a poor environment to any modern electronics like crystals or LDO regulators.

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Jepael wrote:
Or you don't draw any current from it so output is unstable.

Or your capacitors are too far from the regulator, and the output oscillates.

Anyway the breadboard is a poor environment to any modern electronics like crystals or LDO regulators.

The capacitors are not far. They are right next to the VCC/GND pins, since they are decoupling caps.

Yeah, let me put a load (a resistor in series with a LED) on it, so that some current draws and see how it goes.

My multi-meter is fine.

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But the 100uF capacitor is surely too large as per USB bus specifications for 5V usage?

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As far as I can see from the datasheet,
the L78L33 is neither a "LOW DROP" nor
a "PRECISION" regulator. Nevertheless it
should deliver results as given in the spec.

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I agree to that - I did not know something like L78L33 existed but I would have preferred LD/LM1117 in fixed 3.3 or adjustable versions.

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My concern is with the dropout voltage. The datasheet quoted says 1.7V typical (ie, it could be higher). For a 3.3V output that means 5V input minimum (typical). That's pretty borderline for a USB port, which can easily be 4.8V or 4.7V due to voltage drops in the cables & connectors, etc. For this circuit to work reliably, you really need to choose a low dropout regulator.

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I use the much better LE33CZ for that sort of application. It works properly with 5V input, as it's an LDO.

Leon Heller G1HSM