NEED INFORMATION- LMH6643MA/NOPB

Typical power dissipation equals typical supply voltage x typical supply current.  Max power dissipation is harder to calculate because it requires knowing load current.

The theta JA is in the datasheet.  Which package are you using?  I may be able to find more thermal data.

Hi,

Can I power LMH6643MM from 0 to 5V? Datasheet recommends >5.5V. How the performance get affected if it is powered from 0 to 5V?

Kindly advise!

Regards | Sinoj

 When I looked up the LMH6643 datasheet, I found the Operating voltage (Page 3) is specified at 2.7 to 12.8V.  There are EC tables for 3V, 5V and +5,-5V supplies.  

Were you, perhaps, looking at another part with a similar number?   

http://www.ti.com/lit/ds/symlink/lmh6643.pdf

Hi Loren,

Thanks for your reply.

Sorry for misleading. I copied a wrong part no. above. The actual part that I looking was LM7171BIM. In my schematic the output of LMH6643MM is fed to LM7171BIM which caused the typo error.

Kindly assist!

Regards | Sinoj

Hello Sinoj,

I looked over the datasheet for the LM7171.  It looks like the 5V supply condition was deliberately excluded.  I am not expert on that part, so I can only interpret what is on the datasheet, but it looks like the amplifier needs 2V from each rail for the output devices.  This means that with a 5V supply the outpus would have only 1V of swing available.  If the 5V supply is not accurate the output swing is even less.

What does not show on the datasheet is where the internal bias circuits cease to function.  At some voltage the device will stop working as the internal bias currents collapse.  This voltage will be process and temperature dependent, so testing one device at room temperature will not show this behavior.  So, it would require a lot of testing to requalify this part to operate below 5.5V.  

I would recommend using a different part.  An amplifier like the LMH6523 is current feedback so it has good slew rate and it is specified to work at 5V supply condition.  It does not have the slew rate of the LM7171, but I do not thing the LM7171 would have good slew rate on 5V supply either.  The LM7171 is designed to work best at high voltage conditions.

If you can give me more information on your application I would be happy to try to find a part that would meet your needs on a 5V supply. 

We also offer Simple Switcher power supplies that could provide a -5V supply for the LM7171 from a single 5V supply.  I like the LMR70503 because it requires very few external components.  The LM2611 requires more components, but can supply more current. 

Hello Loren,

Thanks for your advise. We solved the problem by powering it from 10V.

Here I have a different query. Attached the circuit section which is part of a legacy design which I'm working on. We use LM7171BIM on the monochrome video path. During a recent review we noticed that 7171 is wrongly configured, but fortunately it was giving the proper output (not from a design perspective, but from the functional perspective).

The video contains approx. +750mV DC offset (changes w.r.t camera) which we remove using R93 and D29-1. The D29-1 stage introduces a gain of +2.63 (measured). D27.6 gives a gain of +0.39. Why D27 gives a non-inverted output with gain 0.39? How it is functioning in this configuration? When I changed D27.4 from GND to -10V (now D27 works from -10V to +10V) D27.6 was giving a gain of  -1 (inverted), which is expected as per R141 (750R) and R140+R91 (750R+5R1).

Can I keep 7171 same as in the circuit (0 to +10V)? Functionally it is OK, but how?

Regards | Sinoj