7660 voltage inverter. No output current limit?

[carl0s]

Hi. I need to generate -5v for another LCD project. I have found the 'industry standard 7660' chip - made by all sorts of companies - TC7660, TP7660, ICL7660.

Anyway, the datasheets don't seem to list a maximum current that they can supply. The only current thing listed is 'supply current', but as it is in the region of 80-200 micro amps, I am presuming that this is the current that the device itself steals, as opposed to what it can supply. Or am I misunderstanding? Surely most things (LCDs) are going to need more than 200 micro-amps ?

Here's a datasheet: https://docs-emea.rs-online.com/webdocs/14f3/0900766b814f3ce5.pdf

[oPossum]

The output voltage droops as you increase the load. So the maximum load is determined by how low a voltage you can make use of. See figure 2-7 in the data sheet.

[Ian.M]

The abs max ratings:

Output Short Duration (VSUPPLY <= 5.5V)...............Continuous
Package Power Dissipation: (TA  +70°C) (Note 2)
 8-Pin PDIP ..........................................................730 mW
 8-Pin SOIC..........................................................470 mW

+ note 2 (derating with temperature),

Note 2: Derate linearly above +50°C by
5.5 mW/°C.

in conjunction with FIGURE 2-7: Output Voltage vs. Output Current, tells you all you need to know.   Fig. 2-7 has curves for various input voltage from 2V to 12V (when unloaded Vout=-Vin), and as its a flying capacitor charge pump, neglecting the chip's internal operating current Iin=Iout.   Therefore the difference between the absolute input and output voltages mutiplied by the load current is the dissipation.  Below 5.5V you cant exceed its dissipation limit, so its short-circuit proof.  Otherwise, check fig. 2-7 against max dissipation, derated for ambient temperature to see if your proposed supply voltage and load current is permissible.

If there's any chance of user idiocy at supply voltages above 5.5V, a 50mA polyfuse on the output would be advisable.

[carl0s]

Ah of course, since it's not a fixed voltage device they wouldn't just specify a maximum current output would they.

Thanks for the help!

[David Hess]

Usually just an output voltage versus output current graph is included for a given operating frequency and charge pump capacitance.  The series resistance of the switches combine with the operating frequency and charge pump capacitance to make an equivalent output resistance.

The supply current specification just refers to the operating current without any load.

If higher output currents are required, then there are higher performance charge pumps than the old 7660 available.  Linear Technology (now Analog Devices) makes a bunch.

[JS]

Quick look shows curves up to 100mA, with the sweet spot at about 5mA and working reasonable under 20mA.

JS