[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

 

[John Popelish]

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.

The formula given for the on time dependent on the resistor connected
to pin 6 says that the time is proportional to the value of the
resistor and inversely to the input voltage. This implies that the
full input voltage is applied across the resistor. So I suspect that
pin 6 connects to an internal current mirror that has only a diode
drop to the ground pin (which they neglect ot include in the
formula). What ever current you pour into this pin, the mirror pulls
down on the internal 'on time' timing capacitor. So you are probably
seeing a reasonable voltage on pin 6.

 

[Spehro Pefhany]

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

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.


Best regards,
Spehro Pefhany

 

[Jamie Morken]

Hi,

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

 

[Spehro Pefhany]

Hi,



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

No, sorry. I followed the data sheet and had absolutely no problems
with R1=200K, my choice of inductor and capacitors that fit their
recommendations (series 1-ohm resistor). I have the evaluation board
too, somewhere.


Best regards,
Spehro Pefhany

 

[Jamie Morken]

Hi,

No, sorry. I followed the data sheet and had absolutely no problems
with R1=200K, my choice of inductor and capacitors that fit their
recommendations (series 1-ohm resistor). I have the evaluation board
too, somewhere.

Did you use 100K for R2 as well? I did. With the 200K R1, and 100K R2,
and a 12V output voltage setting, will these chips be able to handle 14V
to 75V with the 200K R1 and 100K R2 values, or is it best to set these
values for R1 and R2 based on a static input voltage?

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

 

[Spehro Pefhany]

Did you use 100K for R2 as well? I did. With the 200K R1, and 100K R2,
and a 12V output voltage setting, will these chips be able to handle 14V
to 75V with the 200K R1 and 100K R2 values,?

That's approximately what I did in that particular product design-
it's almost the same as page 2 on the data sheet.

or is it best to set these
values for R1 and R2 based on a static input voltage

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. ;-)


Best regards,
Spehro Pefhany

 

[Jamie Morken]

Hi,

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.




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