lm5007 buck switcher troubles

[Jamie Morken]

Hi all,

I am using the LM5007 from national:
"http://www.national.com/pf/LM/LM5007.html"

I designed the circuit identical to the circuit shown on page2 of the
datasheet, but it is not working. I believe it is something to do with
the 200Kohm R1, as when I measure the voltage on pin6, it is under 0.7V
which means the chip is in shutdown mode. I tested the chip with input
voltages from 12V up to 75V and it is in shutdown mode no matter what.

I disabled the shutdown mode by increasing the voltage on pin6 but it
fried the chip too. Any ideas on this? I have ordered the evalboard
from national to see if it works! I designed my board the same as
the eval board.

cheers,
Jamie Morken

 

[Jamie Morken]

Hi,

Spehro Pefhany wrote:

You need the resistor R1 and it has to be roughly 200K for it to work
properly (depending on maximum input voltage). It just looks like a
pullup in the first schematic, but it's actually doing more- it sets
the output switch on-time.

Hmm.. any ideas why it isn't working then? pin6 is below 0.7V so it
is in shutdown mode, but I am not sure why..

cheers,
Jamie

Best regards,
Spehro Pefhany

 

[Jamie Morken]

Hi,

Spehro Pefhany wrote:

Best read the data sheet carefully. ;-) It was almost a year ago,
don't remember the details (it gets into both continous and
discontinous mode, depending on load). Power dissipation was the main
concern.


I noticed an error on my design, I forgot to hook pin4 to ground, I did
this now but I am still getting no output voltage, maybe I fried the
chip by not having this pin grounded?

cheers,
Jamie


That's the only ground pin. It certainly won't work very well without
it. ;-)

I had to replace 4 of 6 chips as not having this pin grounded fried
them, but they seem all to be working fine now!

cheers,
Jamie

Best regards,
Spehro Pefhany