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lm5007 buck switcher troubles

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.

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John Popelish
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John Popelish
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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

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

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

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Hi all,

I am using the LM5007 from national: "

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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

Reply to
Jamie Morken

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

Reply to
Jamie Morken

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

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. ;-)

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
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, 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

Reply to
Jamie Morken

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

Reply to
Jamie Morken

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