HI all,
I have been racking my brain the last two days trying to design a simple linear/buck constant current regulator for driving LEDs. I've looked at everything I can think of - discrete ICs from all the major players (TI, Diodes, Linear Tech etc) to building my own using MOSFETs and so on. I feel my knowledge is borderline hopeless, so I've come here to see if anyone can help out.
Now before I go on, yes I have read the sticky in this very forum on driving LEDs. I think I should list my requirements to show you why what's listed in the sticky won't work, or at least show me that I don't know how to make them work for me.
- Single Lithium Ion rechargeable power source (4.2V fresh, 3.7V nominal, 2.7V flat)
- Single HB LED (XPG, XPE, Nichia 219)
- Maxium current of 1000mA (over 800mA is desired)
- Maximum Vf or 3.3V will be needed
- PWM control. I will be pairing the driver with an uC to control brightness.
So as you can see all I really need is a linear LED driver. I stumbled across a circuit that meets all my requirements except for the current output (this circuit only manages 400mA). All the circuit contains is a 400mA low drop out linear regulator (PJFQ), an output capacitor and a PIC (Microchip 12F629). Photo of the circuit is below:
How does this circuit function? The confusing part for me is that 99% of the LDOs I find only have an enable pin (which is not appropriate for PWM control) and secondly LDOs have a fixed output voltage.
It looks like the LDO output pin is connected to the +ve terminal of the LED, but then the LED -ve terminal is connected to ground...LDOs output a constant voltage so how can this be working? I can't see how the uC is connected up as the signal traces are too tight to differentiate
Any help would be highly appreciated as I don't think I can look at anymore datasheets!
Thanks,
- Matt
I have been racking my brain the last two days trying to design a simple linear/buck constant current regulator for driving LEDs. I've looked at everything I can think of - discrete ICs from all the major players (TI, Diodes, Linear Tech etc) to building my own using MOSFETs and so on. I feel my knowledge is borderline hopeless, so I've come here to see if anyone can help out.
Now before I go on, yes I have read the sticky in this very forum on driving LEDs. I think I should list my requirements to show you why what's listed in the sticky won't work, or at least show me that I don't know how to make them work for me.
- Single Lithium Ion rechargeable power source (4.2V fresh, 3.7V nominal, 2.7V flat)
- Single HB LED (XPG, XPE, Nichia 219)
- Maxium current of 1000mA (over 800mA is desired)
- Maximum Vf or 3.3V will be needed
- PWM control. I will be pairing the driver with an uC to control brightness.
So as you can see all I really need is a linear LED driver. I stumbled across a circuit that meets all my requirements except for the current output (this circuit only manages 400mA). All the circuit contains is a 400mA low drop out linear regulator (PJFQ), an output capacitor and a PIC (Microchip 12F629). Photo of the circuit is below:
How does this circuit function? The confusing part for me is that 99% of the LDOs I find only have an enable pin (which is not appropriate for PWM control) and secondly LDOs have a fixed output voltage.
It looks like the LDO output pin is connected to the +ve terminal of the LED, but then the LED -ve terminal is connected to ground...LDOs output a constant voltage so how can this be working? I can't see how the uC is connected up as the signal traces are too tight to differentiate
Any help would be highly appreciated as I don't think I can look at anymore datasheets!
Thanks,
- Matt
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