Small MOSFET power buffer  - Page 1
Forum section: electronics / projects

author: Sniper1
author_rank: Regular Contributor
timestamp: November 27, 2022, 05:55:19 pm
content: So here is some context:I need some negative voltage in order to use a 5V LCD with a 3v3 MCU (STM32 ) and after a bit of research i found out that charge pumps are a easy way to do this and for the LCD i would need 1-0.5mA at -1V or a bit less , this should be ok/doable with a GPIO pin. BUT i also have some op amp circuitry and filters and it would be nice to be able to get 0V there too but that circuit needs like 13mA according to LTSPICE.So i tried to make a CMOS buffer in spice and failed hard , multiple op amps in parallel seem to work , so does a CMOS 555.repo:https://github.com/immortal-sniper1/Progrmable_Active_Loads/tree/master/Small_load_V1/simulationsSo can anyone give me a hint of what i am doing wrong?I was thinking of using some TO-92 like:https://eu.mouser.com/ProductDetail/Microchip-Technology-Atmel/VP2106N3-G?qs=NZF3WWV0eEGNFUSyHi4JQQ%3D%3Dhttps://eu.mouser.com/ProductDetail/onsemi-Fairchild/2N7000BU?qs=k2x4EL1%2FKj6oeXMHAfSm5A%3D%3DAs for the current i saw in SPICE the peaks are bout 280mA but the RMS is about 60mA and this makes me wander if op amps in series will realistically work.NOTE: since the PWM will come from a STM32 i can vary the duty and f but from what i read 50% is best and if f is great it is better as in lower cap values and since i can go as fast as 1MHz why not?Also i have a 12V boost so if it is the only way i can also borrow from that switching node and get -12V but then ill need some negtive regulation back to -1 -2 V...Am i missing something?PSis there any other simple SMPS inverting design that i can DIY with 1-2 GPIO in order to get a 15-20mA negative rail?

author: Shay
author_rank: Regular Contributor
timestamp: November 28, 2022, 12:36:47 am
content: Quote from: Sniper1 on November 27, 2022, 05:55:19 pmThere are plenty of ways to generate negative rail, using somthing like ICL7660 or MAX1044, or isolated dc-dc converter:https://he.aliexpress.com/item/32840613546.html?spm=a2g0o.productlist.main.1.1a3eNWhgNWhgUf&algo_pvid=8b6aaad7-833b-40b3-950c-c43d84eb6b78&algo_exp_id=8b6aaad7-833b-40b3-950c-c43d84eb6b78-0&pdp_ext_f=%7B%22sku_id%22%3A%2265123770336%22%7D&pdp_npi=2%40dis%21ILS%212.62%212.1%21%21%21%21%21%402100b88516695944540848124d0749%2165123770336%21sea&curPageLogUid=26lT8rWjUOWNAs long as you have a uController that can generate a square wave, you can make a charge pump:the frequency of the square wave shall be as fast as possible in order to reduce ripple at the output. I recommend playing with the capacitors value.The average current is about 50mA. each GPIO pin should be able to generate ~40mA max. so either utilize 2 charge pumps and connect the outputs together, or parallel the GPIO pins.

author: Sniper1
author_rank: Regular Contributor
timestamp: November 28, 2022, 11:31:14 am
content: A cording to this:https://eu.mouser.com/datasheet/2/389/dm00440565-1799229.pdfIt is 16mA per pin then again i am not sure if it is constant or peak. Not sure how i can turn 2-3 GPIO on at the same time on a stm32but also if i reduce the caps current and negative voltage will fall and since i need only -1V that may be a safer route.BTW i also have 1-2 free op amp would puting them in parallel end up with better overall?https://www.ti.com/lit/ds/symlink/lm2904lv.pdf?HQS=dis-mous-null-mousermode-dsf-pf-null-wwe&ts=1669635009383&ref_url=https%253A%252F%252Fro.mouser.com%252Fit seems to have 40mA per channel so maybe use 2 like a buffer ?Ill simulate a bit more anyway .THX for the advice

author: mawyatt
author_rank: Super Contributor
timestamp: November 28, 2022, 03:00:31 pm
content: Quote from: Shay on November 28, 2022, 12:36:47 amOr one could just use a hex inverter like 74ACT04 (74HC04) as a buffer by paralleling all the inverters.Best

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 28, 2022, 04:04:43 pm
content: Use a 74hc14, use first gate as free-running oscillator, rest of the pins as current buffers.This will make about 17V, adjust the doubling stages to the required voltage. Each stage adds ~4.3V.

author: Sniper1
author_rank: Regular Contributor
timestamp: November 29, 2022, 07:48:43 am
content: THXvoltage is not that big og a concearn if it is under -0.7V but current is that is why i am useing only 1 stage but yea 74 series chips are a good idea + i can make all 6 be buffers and adjust the PWM f and duty with my STM32