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Boscoe 28th February 2012 08:12 PM

LME48910 biasing...
 
The schematic is in the datasheet here.

If someone good help me with some of these it would be great!

1. I wonder about the Vbe multiplier they have displayed and talk about in the text. It says that 2.8mA is a good value to flow from BIASm to BIASp but out of interest how would I work this out not know the voltage presented by these two pins? I know it's knot a simple as putting a resistor there but I would like to know more.

2. Also they talk about a bias formula which I haven't got a clue about.

3. I prefer to use FETs so could I just use diodes and forget the Vbe because I don't have to worry about thermal runaway? I want to use IRFP240 and it's partner. How would I implement this?

Thanks
Boscoe

qusp 29th February 2012 04:57 AM

by using lme49830

h_a 29th February 2012 06:20 AM

Hi Boscoe,

Quote:

It says that 2.8mA is a good value to flow from BIASm to BIASp but out of interest how would I work this out not know the voltage presented by these two pins?
Datasheet says

The LME49810 has two dedicated pins (BIASM and BIASP) for Bias setup and provide a constant current source of about 2.8mA.

so there's a current source and whatever voltage difference you want you can obtain by choosing suitable total resistance in your vbe-multiplier ;)

Quote:

because I don't have to worry about thermal runaway?
Fets are sensitive to thermal runaway! Their zero tempco point you refer to is at several amps for vertical types as the IRFP240!

Hannes

qusp 29th February 2012 06:32 AM

yeah its only laterals that are not as sensitive to thermal runaway

astankov 29th February 2012 06:44 AM

Quote:

Originally Posted by qusp (Post 2927416)
yeah its only laterals that are not as sensitive to thermal runaway

Absolutely true!

IRFP240/9240 are very sensitive to thermal changes and if you don't fit some thermal feedback be ready for nasty surprise...:redhot:

carlmart 29th February 2012 11:42 AM

On the LME48910 datasheet there's also an application board using those very same mosfets you want... with VBE.

Boscoe 1st March 2012 06:59 PM

So with negative temp coefficient fets I can just use a resistor to determine volts drop from the constant current source?


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