# Supply current of negative rail

I'm using an LTC1983ES6-5 to provide a negative rail for both a MAX4311 and a MAX4395. LTC1983 provides up to 100 mA, but I cannot find in the other datasheets how much current is required for the negative supply rail of the components.

What parameter(s) I should look to?

The datasheets for both ICs have a section called "output current." Both datasheets give the conditions for that output current:

MAX4311:

That's 95mA into a 30ohm load when operated at 5V, with 2.5 V output.

MAX4395:

Again, 95mA. But, into a 75ohm load operated on +- 5V.

You have to add in the quiescent current, of course. Also given in the datasheets.

It looks to me like your LTC1983 is a little on the weak side (100mA opposed to the 200mA both amps can draw,) at least if you are driving anywhere near the rated loads - and I expect you are, since both ICs are intended to drive video signals into typical video cable impedances.

You know your circuit better than we do. If you are driving lower loads, you might get away with just one LTC1983. If you are driving typical video signals, you'd do better with two LTC1983 (or a single converter that can supply more current.)

• Yes, I'm driving standard 75-ohm video signals. About MAX4311 because there's no indication about dual-supply, am I wrong to assume (just as rule-of-thumb) a negative current slightly less than the half of the total (95 mA) ? – Mark Apr 9 at 9:24
• That assumption is incorrect. It will draw that current from both supplies. – JRE Apr 9 at 9:30

The datasheet does tell how much the chips itself use. It's the quiescent current section. But what answer you really want depends on how much current is used by the loads connected to these chips. Only you can answer this.

You can start by assuming the supply current for these ICs flows from the positive to the negative supply. Thus all positive supply current must be sunk by the negative supply.

This assumption is not correct if the ICs are sinking or sourcing current on other pins. Current from other pins is generally directed to either the positive or negative supply. Sometimes current is directed from one pin to another (non-supply) pin.

I haven't looked at the data sheets but you should be able to find the supply currents in there.

• This assumption is not correct if the ICs a sinking or sourcing current on other pins and I haven't looked at the data sheets don't make this answer helpful I think. – Huisman Apr 9 at 8:20