Resistor Values for VR feedback Pin? - Page 1
Forum section: electronics / beginners

author: vol.2
author_rank: Contributor
timestamp: November 11, 2024, 04:46:14 am
content: How do I calculate the values for the voltage divider on the FB pin of voltage regulator IC?I am using the LT1170, can be foundhttps://www.analog.com/media/en/technical-documentation/data-sheets/117012fi.pdfThe sample diagram has an output voltage of 12V and uses a divider of 10.7k and 1.24k. That seems to suggest that I can set the output voltage by changing R1 to give me ~1.24V on the feedback Pin of the VR. Does that sound correct?I want 13.5V output, and the input is an unregulated full bridge rectifier that fluctuates somewhat under load, but it's nominally 13.5V (or a bit higher) when loaded in typical operation. R1=12k, R2=1.21k gets close to 1.24V.Also, is the circuit going to be able to regulate at 13.5V if it dips below that or goes above momentarily?TIACode:VIN ~13.5V ___o----------o----------UUU--------o o---------o--o VOUT 13.5V| L1 | | |C2 || | 50uH | | |0.1uF o--||--o | \] | || || | | .--|]--| C4 || | .-------------. | | /]+ | 1000uF || \] | | | | | | 25V |o--|]--o | LT1172 | | | | |C1 | /]+ | | | | | | |220uF | o----|5 Vin | | | _|_ D1 |25V | | | | | /_\ 1N5821 |o---. | | | | | || | | VSW 7|--o--|------o || --- C3 | | | ___ || --- 1uF | | | o--|___|--o| | | | | | R1| | | FB 3|-----|------o 12k?????| .-. | | | || | |R3 | | | .-.| | |1k | | | | |R2| '-' | | | | |1.21k???| | | | | '-'| '-------| | | || | | | || o--|1 GND | | |o--------|--|6 E1 | | || o--|8 E2 | | || '-------------' | |'---------------o---------------o------'|GND

author: PGPG
author_rank: Frequent Contributor
timestamp: November 11, 2024, 03:10:14 pm
content: Quote from: vol.2 on November 11, 2024, 04:46:14 amIt is step-up converter.See at schematic. When input voltage is too high it goes through L1 nad D1 directly to output and IC can't make it smaller.

author: vol.2
author_rank: Contributor
timestamp: November 11, 2024, 06:55:42 pm
content: Quote from: PGPG on November 11, 2024, 03:10:14 pmThanks, I sort of figured that out since yesterday, but I'm having trouble figuring out a topology that will allow the voltage to straddle the input at 13.5V. I found maybe a SEPIC topology would be a good one, but I don't know how I'd implement it with this IC (Analog Devices lists the LT1172 as a SEPIC capable IC).I found other SEPIC design documents from TI, but they have different Pins from what I can see and so no easy way to match it up.Any recommendations?

author: tooki
author_rank: Super Contributor
timestamp: November 11, 2024, 07:40:18 pm
content: Quote from: vol.2 on November 11, 2024, 04:46:14 amLiterally the first line in the electrical characteristics table (page 4) of the datasheet is the reference voltage: 1.244V

author: PGPG
author_rank: Frequent Contributor
timestamp: November 12, 2024, 12:00:47 am
content: Quote from: vol.2 on November 11, 2024, 06:55:42 pmTI at their page have some tool (I have never used and don't remember the name). If you write there that you want input from 10V to 15V and output 13.5V you will force it to suggest you only solutions that can work up and down. You should find there circuits you need.Sepic with 1:1 trafo works like step-up with output voltage (at first coil) higher than input of a value you need at output and you put at output only these added voltage (at first coil) as you get it at second coil. As voltage at both coils jumps the same delta voltage (because 1:1) you can connect these points with capacitor eliminating this way peaks caused by leakage inductance.

author: tooki
author_rank: Super Contributor
timestamp: November 12, 2024, 09:27:32 am
content: That tool is the WEBENCH Power Designer. And it’s brilliant. I’ve used it for a few designs and they all worked. The PCB layouts cannot always be used unmodified — sometimes the footprints for passives aren’t good — but they’re a really good starting point, and I wouldn’t stray far from the overall layout they suggest.What isn’t immediately obvious is that you can edit the schematic to a point: each component can be edited to select another part or value, and it tells you what the value range is. You can then select a component and re-run the simulations to see the effect.I used this, for example, to test the effect of changing the output voltage (by changing the feedback resistor divider) without changing other components, to find inductor and capacitor values that work for a supply with an adjustable output voltage.