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Old 8th December 2004, 07:24 PM   #11
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The adj pin current results from the operation of the regulator, it is not something *you* need to do anything on. As an example, if you take the simplest regulator, a transistor, where you put Vin to the collector and take Vout from the emitter, the Vou is set by the base voltage. There will always be a small base current to ground, and that is equivalent to the adj pin current. Now, in this example the base current of course heavily depends on the load current, but in the more sophisticated (internally) '317, this is almost constant. This current, going through the bottom resistor of the divider setting Vout, causes a voltage rise on the adj pin and therefor upsets the simple equation that says the Vout is depending on the R1/R2 ratio only. The larger the resistors, the larger the error for the same Iadj.

The minimum load current is something else, in the above simple example it would be the minimum current drawn from the emitter for correct operation. This minimum current doesn't run from the adj pin, but from the Vout pin to ground. Of course, you *can* make R1/R2 so low in value that the current through them is the minimum current, but normally the load current should be sufficient for that.

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Old 8th December 2004, 09:47 PM   #12
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That was very helpful Jan!

Edit: I just thought, what happens if you set the resistor at such a high value that the Base to Ground current is below the minimum that's specified in the regulator's datasheet? Does it cause the 'Base' to start collecting a negative charge?
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Old 9th December 2004, 02:13 AM   #13
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I fogot to ask. The output voltage of the regulator is very high, over 400V. I'm trying to keep to SMD parts for the board. This makes using Tantalum beads and even a lot of electrolytics tricky. I've found one series of electrolytics that are about 10V over the maximum of the regulator, a bit risky.

Ceramics seem to be coming to the front for the output capacitor. I can manage about 1 - 2uF maximum of ceramic capacitance at the regulators output.

The electrolytics are also only available on special order.

I know a lot of people dislike electrolytics, how do ceramics compare?
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Old 9th December 2004, 07:54 AM   #14
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Quote:
Originally posted by eeka chu
That was very helpful Jan!

Edit: I just thought, what happens if you set the resistor at such a high value that the Base to Ground current is below the minimum that's specified in the regulator's datasheet? Does it cause the 'Base' to start collecting a negative charge?
Inserting resistance in any application will not cause the direction of the current to reverse, merely do as it's name suggests as reduce it. Thus in your example you simply choke the base and thus restrict the current available from the transistor. In the case of regulator chips it may stop the regulator working or give spurious results.
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