Resistors in Switching Regulator design...

Hi all,

I have a switching regulator design using National's LM2675, adjustable
version. I have input, ouput caps (Low ESR, 10uF input, 100uF output)
inductor (100 uH) and the feedback resistors to adjust the output
voltage. (Also the diode and the boost cap. as shown in datasheet)

As the datasheet suggests, I have placed 1.5k and 4.75k respectively.
When I measure the resistors when the inductor is not mounted on the
circuit I can read correct values, but when i solder the inductor and
then measure the resistances, I always get a lower value (such as 1.3k
and 2.2k instead of 1.5k and 4.7k)

And the output voltage is not 5V and instead something very low. Around
1.5 - 2 Volt.

I have reviewed my board layout and schematics and they seem to be 100%
same as what the datasheet suggests...
Does anyone have any ideas ?


Thanks,
Antonis

 

[Spehro Pefhany]

On 7 Dec 2004 12:12:35 -0800, the renowned
antonis_konstantinos@hotmail.com wrote:

Hi all,

I have a switching regulator design using National's LM2675, adjustable
version. I have input, ouput caps (Low ESR, 10uF input, 100uF output)
inductor (100 uH) and the feedback resistors to adjust the output
voltage. (Also the diode and the boost cap. as shown in datasheet)

As the datasheet suggests, I have placed 1.5k and 4.75k respectively.
When I measure the resistors when the inductor is not mounted on the
circuit I can read correct values, but when i solder the inductor and
then measure the resistances, I always get a lower value (such as 1.3k
and 2.2k instead of 1.5k and 4.7k)

And the output voltage is not 5V and instead something very low. Around
1.5 - 2 Volt.

I have reviewed my board layout and schematics and they seem to be 100%
same as what the datasheet suggests...
Does anyone have any ideas ?


Thanks,
Antonis

Perhaps a silly question, but have you reversed the two resistors? The
4K75 should go to the output and the 1K5 to ground.


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

 

Perhaps a silly question, but have you reversed the two resistors? The
4K75 should go to the output and the 1K5 to ground.

----
indeed silly,
silly me !

I checked once again and you are definitely right! Now it works great
!

Thanks for reminding..

Antonis