Change LM2575 minimum input voltage to 8 volt

[chihwah li]

Dear all,

The LM2575 has a minimum input voltage of 7V, but that is a little bit too low. I like to put 8 pieces of Nimh AA batteries in series to Get the required input voltage. But 7V would drain my batteries perhaps too low and that is not good. I read somewhere that you should not drain a Nimh battery lower than 900mV. 8 times 0.9V = 7.2V

How can I increase the LM2575 input voltage to 8 volt?

A working schematic for 7Volt input is in the pdf:
http://www.ti.com/lit/ds/symlink/lm1575.pdfhttp://www.ti.com/lit/ds/symlink/lm1575.pdf

 

[Alec_t]

Which version of the LM2575 do you have?
The TI datasheet for the LM2575-5.0 gives specs for supply voltages in the range 8V-40V, but that doesn't mean the chip will suddenly stop drawing current when the input voltage drops below 8V (or 7V). Even in the 'Off' state the chip draws ~200uA. You will need to monitor the battery voltage and disconnect the regulator if the voltage drops too low.

 

[Colin Mitchell]

There is no LM2575-7.

Use any -3.3 -5 or adjustable version and use the adjustable circuit to jack up the voltage to 8v.
Or use a commonly available 7805 or LM 317 regulator in "jack-up" circuit.

 

[CDRIVE]

Odd. My TI data sheet specs 4.75V to 40V input.

Disclaimer: It's Sunday afternoon, raining heavily and I'm working diligently at sampling my alcoholic stores. I'm an expert at this sadly diminishing science.

Chris

 

[Alec_t]

My TI data sheet specs 4.75V to 40V input.

That range is certainly mentioned. The point is (so far as I understand it) that the OP doesn't want to over-discharge his battery, so he'd really like the IC to shut off completely when the input drops to 8V.

 

[CDRIVE]

Oh, I evidently didn't understand his question. Thanks for the clarification Alec.

Chris

 

[CDRIVE]

Even in the 'Off' state the chip draws ~200uA. You will need to monitor the battery voltage and disconnect the regulator if the voltage drops too low.

I have a folder that has quite a few Voltage Monitor chips but they're designed for use with uCs. These chips draw only a few uA. I would think that there must be Voltage Monitor chips available for voltages >5V.

Did you Google it?

Chris

 

[CDRIVE]

Here's one..
http://www.linear.com/product/LTC2960

Chris

 

[CDRIVE]

I just happened to notice that there's Another Topic that intersects this one. Your LM2575 also has an ON/OFF pin discussed in that topic.

Chris