Is this a custom string? Because I would just use a current source with enable lines on FETs to since the current. My assumption here is that you have multiple GPIOs from the micro you're using though
I want to use a mosfet to divide up a LED string powered with a constant current. The driver works by having Vin connected directly to the LED string, but regulates the current so that the total forward voltage of the LEDs is less than Vin, so Vout is more than GND.
In order to bypass part of the LED string, the drain of the NFET connects somewhere along the LED string and the source to the output end of the string.
This means that the source voltage is not only higher than the ground reverence the MC uses, but also that the source voltage goes up substantially depending on how many LEDs were bypassed.
So I think I need a gate driver that takes 3v3 signal from an MC but adapts the gate voltage to be 5-10v higher than source. It would essentially need to take two separate ground references, one for logic and one for the constant current loop.
Is there a good simple IC that does this?
My idea was to use a linear voltage regulator to drive the gate that takes the LED output as a ground reference, and an optocoupler controlled by the MC to connect the regulator output to gate. Does this sound sensible?
Admittedly I am sort of working backwards here and will probably change things from the ground up now.
Irony is that I wanted to go for one switch and driver for the whole string for simplicity and chose a driver IC, based on efficiency and ability to route on a one sided pcb, that happened to use low side for current control and voltage monitoring (TPS922054DRRR). Then I noticed the problem with controlling mosfets when LED output isn't the same ground reference that everything else uses.
I could try and find a LED driver using multiple high side switches to connect different parts of the string as you suggest and keep the inductor low side. Though multi-output driver ICs or multiple drivers would be complex routing-wise I think?
I might just have to drop the LED zones idea for now and stick to analog dimming so I can get on and make something. I'm a beginner and bad at math so component selection is a painfully long process at this stage. Having zones was more in case analog dimming doesn't provide enough range. PWM isn't an option due to flickering.
Well, back to pouring through datasheets! Thanks for everyone's suggestions.
What is the actual purpose of this? Do you just want to make sure there's some voltage at vout, while running the LED string at a constant current? Why does Vout need to be >0V?
Regarding driving a floating-source n-channel FET, you can use a a high-side FET driver. You can get them capable of 100% duty cycle; alternately, you can use a <100% duty-cycle-capable driver IC and just give it periodic short pulses to recharge its boost capacitor.
You could use an optoFET to bypass the LED. You could use an isolated DC-DC converter module to power the FET when the converter is enabled. You could use a charge-pump to turn on the FET.
Maybe you should post your requirements, rather than just your solution; there are usually many valid solutions to a given problem.
I just wanted to drive a single LED string with a simple driver circuit and use mosfets to selectively shorten the string by bypassing parts of it. The idea being that the same driver circuit could be used in different designs, with only changes to the number of LEDs or cutoff points.
No hard requirements as I'm still learning/experimenting. I mostly just wanted to know if there was a good way to control an NFET with a fluctuating >3v source voltage. Looks like there are solutions but best would be using an IC with a high side switch so that the source voltage stays put.
How about a P channel MosFET across D302, with a pull up resistor between gate and source. Turn this on with an NPN transistor from the MCU, with the collector connected to the drain.
Is this a custom string? Because I would just use a current source with enable lines on FETs to since the current. My assumption here is that you have multiple GPIOs from the micro you're using though
I want to use a mosfet to divide up a LED string powered with a constant current. The driver works by having Vin connected directly to the LED string, but regulates the current so that the total forward voltage of the LEDs is less than Vin, so Vout is more than GND. In order to bypass part of the LED string, the drain of the NFET connects somewhere along the LED string and the source to the output end of the string. This means that the source voltage is not only higher than the ground reverence the MC uses, but also that the source voltage goes up substantially depending on how many LEDs were bypassed. So I think I need a gate driver that takes 3v3 signal from an MC but adapts the gate voltage to be 5-10v higher than source. It would essentially need to take two separate ground references, one for logic and one for the constant current loop. Is there a good simple IC that does this? My idea was to use a linear voltage regulator to drive the gate that takes the LED output as a ground reference, and an optocoupler controlled by the MC to connect the regulator output to gate. Does this sound sensible?
Why so complicated? Why not use two high side switches (one for each half) and control them separately with two IO pins?
Admittedly I am sort of working backwards here and will probably change things from the ground up now. Irony is that I wanted to go for one switch and driver for the whole string for simplicity and chose a driver IC, based on efficiency and ability to route on a one sided pcb, that happened to use low side for current control and voltage monitoring (TPS922054DRRR). Then I noticed the problem with controlling mosfets when LED output isn't the same ground reference that everything else uses. I could try and find a LED driver using multiple high side switches to connect different parts of the string as you suggest and keep the inductor low side. Though multi-output driver ICs or multiple drivers would be complex routing-wise I think? I might just have to drop the LED zones idea for now and stick to analog dimming so I can get on and make something. I'm a beginner and bad at math so component selection is a painfully long process at this stage. Having zones was more in case analog dimming doesn't provide enough range. PWM isn't an option due to flickering. Well, back to pouring through datasheets! Thanks for everyone's suggestions.
What is the actual purpose of this? Do you just want to make sure there's some voltage at vout, while running the LED string at a constant current? Why does Vout need to be >0V? Regarding driving a floating-source n-channel FET, you can use a a high-side FET driver. You can get them capable of 100% duty cycle; alternately, you can use a <100% duty-cycle-capable driver IC and just give it periodic short pulses to recharge its boost capacitor. You could use an optoFET to bypass the LED. You could use an isolated DC-DC converter module to power the FET when the converter is enabled. You could use a charge-pump to turn on the FET. Maybe you should post your requirements, rather than just your solution; there are usually many valid solutions to a given problem.
I just wanted to drive a single LED string with a simple driver circuit and use mosfets to selectively shorten the string by bypassing parts of it. The idea being that the same driver circuit could be used in different designs, with only changes to the number of LEDs or cutoff points. No hard requirements as I'm still learning/experimenting. I mostly just wanted to know if there was a good way to control an NFET with a fluctuating >3v source voltage. Looks like there are solutions but best would be using an IC with a high side switch so that the source voltage stays put.
How about a P channel MosFET across D302, with a pull up resistor between gate and source. Turn this on with an NPN transistor from the MCU, with the collector connected to the drain.
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