Electronics > Projects, Designs, and Technical Stuff
Voltage Regulator Bypass Circuit
LittleRain:
I am building a device which runs on USB or Bluetooth with a battery.
My power source goes to a 3.3v LDO regulator, and I want to bypass the regulator just before the regulator starts to dropout.
The plan was to use a voltage detector, the TPS3780, to detect when the battery drops below 3.4v, which will drive a mosfet to bypass the regulator.
The TPS3780 has 2 outputs, so I was going to use the second to put my MCU in a reset state when it drops below 3v.
Problem is my board is getting more and more expensive, and the 2-3$ IC's keep building up.
So I was thinking, what if I just used a 3.4v zener diode connected to the output the battery.
The zener would be attached to the positive input of a comparator, and a voltage divider from the battery on the other input.
When the regulator/battery drops below the zener it will stop flowing, toggling the comparator.
And then I would drive the mosfet with the output from the comparator.
Here is my schematic, only 1 would be connected of course.
I'm also going to switch out the comparator for an open-drain instead, and if you were wondering, the resistors at the TPS3779 value is "?" because I haven't calculated them yet.
Would my zener/comparator circuit work? And is there anything I should take into consideration?
bob91343:
A 3.4 V zener is a nonstandard value. Even if you select from a lot of 3.3 V units, they are terrible regulators, with impedance too high for regulation, not to speak of a somewhat negative temperature coefficient. You will be disappointed.
Siwastaja:
Just pick an LDO with low-enough drop-out, i.e., one which internally does exactly what you are doing. FET based LDOs have everything needed internally, you are duplicating the functionality. Sort for drop-out voltage on parametric search.
Using a lower RDS(on) MOSFET to bypass it only gains you some 0.1V, maybe 0.2V extra compared to a properly selected regulator with very low drop-out. That extra 0.1 or 0.2V would gain you relatively little battery life; your solution have extra parts with extra quiescent current that may eat the benefit.
MasterT:
I see AP7215 states 250mV max drop-out at 100mA. There is an APPLICATION NOTE 1853 from Maxim-integrated that :
--- Quote ---a linear-regulator circuit whose dropout at 100mA load current is only 10mV.
--- End quote ---
LittleRain:
--- Quote from: MasterT on January 25, 2020, 03:14:03 pm ---I see AP7215 states 250mV max drop-out at 100mA. There is an APPLICATION NOTE 1853 from Maxim-integrated that :
--- Quote ---a linear-regulator circuit whose dropout at 100mA load current is only 10mV.
--- End quote ---
--- End quote ---
--- Quote from: Siwastaja on January 25, 2020, 01:26:55 pm ---Just pick an LDO with low-enough drop-out, i.e., one which internally does exactly what you are doing.
--- End quote ---
Problem is I want the battery to be able to go down to 3v, and I'll also be using a CPLD and DC motors, so I'll be using close to the full 500mA from the USB port.
I really want to get as much battery life as possible.
Should I just switch to a 2.5v regulator, and make sure all the 3.3v components can run at 2.5v, and switch them if they cant?
Id have to check and see if the MCU can run at the clock speed I need from 2.5v first though.
The CPLD is the Max II EPM240T100C5, which can run off of 3.3 V/2.5 V or 1.8 V, pretty sure it doesn't matter as well, I'll be running it at 50mhz instead of its full 100/150mhz.
Or should I use the TPS3780 circuit for bypassing?
--- Quote from: Siwastaja on January 25, 2020, 01:26:55 pm ---Using a lower RDS(on) MOSFET to bypass it only gains you some 0.1V, maybe 0.2V extra compared to a properly selected regulator with very low drop-out. That extra 0.1 or 0.2V would gain you relatively little battery life; your solution have extra parts with extra quiescent current that may eat the benefit.
--- End quote ---
Here is an image of simulated battery life using the TPS3780 to bypass the regulator, you do get a bit more life using it.(I didn't take the pic, its from here)
Here is the image without.
--- Quote from: bob91343 on January 25, 2020, 05:52:26 am ---A 3.4 V zener is a nonstandard value. Even if you select from a lot of 3.3 V units, they are terrible regulators, with impedance too high for regulation, not to speak of a somewhat negative temperature coefficient. You will be disappointed.
--- End quote ---
I was wondering about that, I know they have 3.3v but haven't checked about 3.4v.
While writing this I just checked, there is this zener, 3.425v for $0.27 CAD, which I could probably find cheaper.
The point of the zener(D3) is not to regulate though, just run a comparator(U17), which will run a mosfet(Q13).
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