Why I use MCP1640 can not get 3.3v - Page 1
Forum section: electronics / beginners

author: yeoman
author_rank: Newbie
timestamp: March 19, 2023, 01:52:28 pm
content: When I build the circuit on the circuit breadboard. the input voltage is 1.3，but output only 2.0v without load resistance. when I input 3v with two cells batteries，the ouput only 2.8v without load. If I add load for circuit, the output voltage is lowee. So why? Is that any problem with official datasheet? Please somebody can help me?

author: Manul
author_rank: Super Contributor
timestamp: March 19, 2023, 02:10:07 pm
content: You should not build such circuit on a breadboard. Period.The problem is too much parasitic inductances, capacitances, also possibly ground loops, weird feedbacks. Yes, it ispossiblethat you build it and it works. But also very likely that it will be failing in some way. So don't waste your time.Throw away the breadboard and build a circuit on a proper PCB and pay attention to recommended layout / best practices.

author: Peabody
author_rank: Super Contributor
timestamp: March 19, 2023, 02:51:59 pm
content: What resistor values are you using for Rt and Rb? The output voltage is determined by those resistors, and the input voltage must be less than that because it's a boost converter. So if 2.0V is the correct output voltage for the resistors you're using, then an input voltage of 3V won't work. it would have to be less than 2V.

author: bidrohini1
author_rank: Contributor
timestamp: March 19, 2023, 04:06:44 pm
content: I can see from the datasheet that, for 3.3V O/P, the 2nd resistor must be 562 K, 309 is for 5V output.https://ww1.microchip.com/downloads/aemDocuments/documents/APID/ProductDocuments/DataSheets/MCP1640-Family-Data-Sheet-DS20002234E.pdf

author: mariush
author_rank: Super Contributor
timestamp: March 19, 2023, 08:26:30 pm
content: The inductor must have good technical properties - see for example page 16 in datasheet where the highest DCR among the suggested inductors is 0.25 ohm - and must be very close to the chip. We're talking maybe 1cm away from chip at most, on your breadboard you have the inductor around 3 inches away from the chip, so you have 6 inches worth of wire with its own resistance and other flaws.Your input capacitor is all the way to the left when in real world it should be as close as possible to the chip - I would basically put right on the same row with the input voltage pin of that DIP adapter for the chip. It's also ceramic, which means the actual capacitance will vary with the input voltage.If it's a 10uF 6.3v ceramic, with 1-2v on it it's probably only 5uF of actual capacitance. You need to use X5R / X7R and ideally 16-25v rated, or higher.Same for the output capacitor.Feeback wires / traces should above going close to the wires going to inductor and the inductor (route the wires around the other side of the chip if needed)As for those feedback resistors, you don't have to use those exact values ... for example instead of 309kOhm you could use 270k (E12 series) or 300k / 3 100k in series / 2 x 150k in series (E24 series) in the case of 5v out, or 470k/560k for 3.3vThen just use formula to determine the other resistor value, formula is on page 15:RTOP = RBOT x( VOUT / VFB -1)So for example for 5v output and using 470K bottom resistor, Rtop = 270k x (5v / 1.21 - 1) = 811K .... so just use a 820kOhm resistor for the topor for 3.3v with 560k resistor at bottom rtop = 560k x (3.3/1.21 -1) = 967k - not a e12 or e24 value, but you could use 820k + 150k = 970k , both standard common values.

author: Jwillis
author_rank: Super Contributor
timestamp: March 20, 2023, 06:16:01 pm
content: Quote from: yeoman on March 19, 2023, 01:52:28 pmI haven't worked with this particular regulator, but some require a minimum load to operate properly . Check application notes for the evaluation boardhttps://ww1.microchip.com/downloads/en/DeviceDoc/51880a.pdf. Many times the data sheets can be a little misleading at first glance until you check the test procedures and parameters.Check application notes that may be provided. Often they can have clues to potential problems.Try starting with a 3K3 1W resistor for a 1mA load. and recheck the voltage.