Drop-out voltage specification on a linear regulator

Using Arduino General Electronics
1

Hi

I'm having another one of those hesitation moments, hoping for a quick piece of practical advice.

The datasheet on the L78L33 3.3V regulator states that it has a drop-off voltage of 1.7V

Does that mean 5V is the minimum acceptable voltage, or is the voltage at which the regulator has already become unstable? I have a couple of these so was hoping to use one up in a project that needs 5V and 3.3V (to run a Nokia 5110 LCD only) so wondering if I need to run VCC into this L78L33 rather than taking the 5V output of the L7805 as the input to this regulator as I would with lower drop-off voltage types where the maths is more obvious.

Thanks
Geoff

2

Does that mean 5V is the minimum acceptable voltage

Yes

3

My understanding is that there will be a minimum gap of the drop-out voltage. So if you put in 5V you get out 3.3V. If you put in 4.9V you get out 3.2V and so on. I'm not sure that it is necessarily unstable at that point.

Figure 10 on page 18 seems to suggest that the drop-out amount varies by the current drawn.

4

The drop out point is the point where the regulator stops regulating. Yes its exact value depends on the current but it is not guaranteed that the output will reduce in step with the dropping input.

5

Thanks guys. Really appreciate the rapid sanity check you provide.

I'll stick with the L78L33 then - they're also quite a bit cheaper than a low drop out version so will make sure I keep them about too.

Cheers ! Geoff

6

At light load, your voltage regulator drops far less than the specified level in the datasheet so you are fine.

Also, the nokia lcd runs fine at 5v.

7

I’ve tried L78L33ACZ (different ICs from one tape,TO-92) with V(in)=5.0V with no success – only 3.0V output (multiple measurements done by oscilloscope and multimeter), with no load at all. With V(in)>5.3V output is stable, V(out)=3.3V. Defective IC?

In datasheet http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00000446.pdf there are no performance graphs for L78L33, and in the table “Table 13. Electrical characteristics of L78L33AB and L78L33AC” test conditions specified only beginning from V(in)=5.3V. Weird…

8

Defective IC?

Did you use the capacitor values given on page 4 of the data sheet, for low drop out it is important that these are followed.

9

Grumpy_Mike:

Defective IC?

Did you use the capacitor values given on page 4 of the data sheet, for low drop out it is important that these are followed.

Yes, I use C(in)=0.33uF and C(out)=0.1uF.

I've found specification for L78L33ACZ at http://uk.farnell.com/stmicroelectronics/l78l33acz/ic-v-reg-3-3v/dp/1467768: Supply Voltage Min = 5.3V, Dropout Voltage = 1.7V. And where is the rest 0.3V? :astonished:

10

On the datasheet, 1.7V is the typical dropout voltage, not the maximum.

PS - low dropout regulators are not expensive, take a look at http://uk.rs-online.com/web/c/semiconductors/power-management-ics/low-dropout-voltage-regulators/?sort-by=default&sort-order=default&applied-dimensions=4294511048,4294511483&lastAttributeSelectedBlock=4294957120 or http://uk.farnell.com/microchip/mcp1700-3302e-to/ic-v-reg-ldo-250ma-to-92-3/dp/1296588?Ntt=MCP1700-3302E/TO.

11

dc42:
On the datasheet, 1.7V is the typical dropout voltage, not the maximum.

Yes, and "blank" value for maximum dropout voltage :wink:

Thank You for MCP1700, it's a good option for me!