lm2678 variable DC DC converter

Hi Gehan,

We've received your schematic. We will get back to you shortly with feedback.

Thanks,

Anston

Hi Gehan,

What you've illustrated looks correct in concept but we have several comments and recommendations.

a) Make sure power supply connections are correct on all op-amps.

b) The next thing we would do is to place a voltage divider from the output of U1:B into the FB of U3.

c) There should be more high frequency bypass Capacitance between C1 and C3.

d) Gain BW product of LM358 may be too slow but then most dual op-amps are pin compatible.

e) The net impedance of Pin6 of U3 is high, so the applied voltage divider should be close to U3.

Is this supposed to be a constant-voltage/constant-current type of supply with a defined horizontal axis current limit and vertical axis voltage limit? or is this a constant-power application with an angled power load line? This looks like the latter.

Thanks,

Anston

Hi thank you for your reply. 

This is supposed to be a variable voltage power supply with an adjustable current limit.

BTW if I want to use the LM2678 as a tracking pre regulator for a linear supply how should I wore the connections?

Is it like this?

how do i select the R1 and R for a particular dropout voltage?

Hi Gehan,

This schematic won't work because the internal reference voltage of the lm2678 is tied to ground. We think you need a differencing amplifier stage that subtracts Vc1 and Vout to create a feedback signal.

Thanks,

Anston

Hi thank you for the reply !!!

isn't there a method where I only use resistors ?

Thanks