0
\$\begingroup\$

I have built a 3.7 - 5V boost circuit using the LM2577-ADJ Step-Up voltage regulator.

Below is a copy of the schematic. enter image description here

I have followed the datasheet (p2) and have calculated Vout as follows:

Vout = 1.23(1+R1/R2)

with values chose for R1 & R2 as:

Vout = 5.05V =1.23(1+5600/1800)

However I have constructed this circuit and with an input voltage of 3.84V (provided by a single cell LiPo) I have an output voltage of 3.56V

Below is a picture of my board layout:

enter image description here

Any help on this would be great, i've checked all the connections for shorts/blown components and I am a little bit stuck.

Cheers

\$\endgroup\$
3
  • 3
    \$\begingroup\$ Compensation should be done with a series RC block instead of a single resistor, R4 (RC block is preferred as shown in the datasheet). The circuit will not be stabilized without proper compensation. \$\endgroup\$ – Rohat Kılıç Apr 26 '17 at 14:36
  • 2
    \$\begingroup\$ Regarding pin 1, the comp pin: in the datasheet there's an R-C to ground. Your schematic only has the resistor. I think this might upset the internal DC level on this pin making the chip think the output voltage is too high and basically switching off. Then what you see at the output is basically the current flowing through the inductor and diode. \$\endgroup\$ – Bimpelrekkie Apr 26 '17 at 14:37
  • 1
    \$\begingroup\$ @RohatKılıç, you should post an answer. \$\endgroup\$ – The Photon Apr 26 '17 at 16:00
2
\$\begingroup\$

Your compensation network is wrong.

As shown in the typical app in the datasheet, compensation should be done with a series RC block instead of a single resistor (R4 in your circuit).

Without proper compensation, the circuit will not be stabilized.

\$\endgroup\$
2
  • \$\begingroup\$ What a thing to miss! I've ordered some caps which will turn up tomorrow. Ill post back/accept with results. \$\endgroup\$ – GPPK Apr 26 '17 at 17:46
  • \$\begingroup\$ Turns out, that although this was missing it doesn't particularly affect the output. The main problem was the manufacturers had put the LM2577 on the wrong way round. Once I swapped that it started working with or without the RC circuit, although I am using very little power so I suspect if I was using more it may cause issues. \$\endgroup\$ – GPPK Apr 27 '17 at 15:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.