schottky diode question

in

Sorry, I forgot one other question. The datasheet says to sequence V+, then Vl, V-, then logic ins. If the V+ was 9v, and Vl was 5v, and I had a power supply that went:

Thanks again

The need for extra diodes suggests that this part has SCR latchup problems. If it does, other things, like signal spikes, could also latch it up... and then it will fry.

Try to find somebody else's part. Maxim tends to be bad news, in general, anyhow.

Why do you need a 5 ohm analog switch?

John

in

I'm using it for switching different guitar effects pedals in and out of my signal chain. I suppose I don't need 5 ohms, but I have a couple here and thought I might as well use them, rather than order other parts.

Thanks

then

power

If the 7809 and 7805 are in series (IOW, output of 7809 feeds input of

Thanks

Yes, but the 5V could become available to the circuit _before_ the 9V output cap is fully charged.

Ed

Hi Tempus

This is what is happening...

The FIG in the Maxim datasheet shows internal diodes from the INPUTS to the V+ line. If your input voltages are brought up before the V+ line, these diodes will conduct, leak current into the chip and the chip may latch up.

What the diagram does not show is that there is probably a similar diode between VL and V+. If you bring up the VL line before V+, then this diode will, like the others, conduct.

You need to bypass this path with a diode with a lower conduction voltage. The internal diodes are normal silicon diodes with a drop of 0.6V. A Schottky diode has a drop of about 0.3V, so this will conduct before the internal diodes.

The 2 diodes in series with the supply pin stop the voltage from the VL line getting back into your V+ supply. When the V+ line comes up, these series diodes will conduct and take over the powering of the chip from the (VL to V+) diodes.

In the power supply configuration you are suggesting, all will be OK. The V+ line (15V) will come up first, the 7905 will take some time, but will come up second then the 7805 will follow.

You then need to just supply V-

Hope this helps

Regards

the

line

V+

Thanks for that Bill - that's much clearer now. Could you clarify something though:

V+

I assume you mean 7809, not 7905 here?

So I don't need to worry about the protection diodes? Also, since I'm using this device to switch audio signals, I need as close to the supply voltage as possible. Would the diodes in series with the supply pin reduce the voltage swing by another 0.6v?

Thanks

Hi Tempus

Ah! you spotted my deliberate mistake. Yes I meant the 7809.

The diodes in series with the supply will limit the signal swing to 1.2V less than the supply (0.6V down from the top rail and 0.6V up from the V-). A doube whammy

Bill

-- Bill Naylor

Maxim have a free sample request service - no questions asked, but in production are difficult to get hold of.

If you only want a few, they are a good company supplying good components. If you are going into production, you had better order well in advance!

-- Bill Naylor