rhd
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Something occurred to me this evening -
I was testing out the MIC29312 IC in a DDL-like configuration. Jufran88 was ambitious enough to put together a PCB based on my suggestion that it would be a good IC to test out. The benefit to this IC, in theory, being that it has an insanely low dropout of 0.35V at 2A current. I won't jump too far into my findings with that IC in the end (Although Jufran88 - your PCB design works great now! The setup just doesn't give us the low dropout we were hoping for). Rather, this thread is really about the fundamental limitation we're encountering in ALL implementations of DDL drivers.
Specifically, DDL drivers use voltage regulators like the LM317, LM1117, and more recently the 1084, 1085. Now, some even fancier ICs like the MIC29312 are starting to poke their heads up. However, the design of the circuit itself is the new limiting factor.
Why? Because our DDL circuit relies on the fact that these voltage regulators work by maintaining a drop of 1.25V across two of their pins. As we change the resistor value between those two pins, the regulator needs to pump more (or less) current across the resistor, in order to engage Ohm's law in a way that produces a 1.25V drop. By that process, the current moves (in an LM317 setup) out from the "out" pin, across the resistor, dropping 1.25V, at which point the drop is measured at the "Adj" pin, and the current then continues on to our laser diode. The fundamental limitation, is that this 1.25V drop must ALWAYS occur, and will (in a DDL circuit) do so in the path of current that flows to our diode. In other words, our DDL circuits will never drop LESS than 1.25V, even with an imaginary IC that could achieve 0V dropout.
So, what's the solution?
The solution I would like to toss out for discussion is not a perfect one. However, short of finding an IC with a lower-than-1.25V reference voltage, this may be out next best shot at improving the DIY linear driver space. For some time, users here have been building 405s, 445s, and even 650s with cheap LED drivers. Yet cheap LED drivers aren't typically constant current sources, they're generally constant voltage sources.
There are drawbacks to a constant voltage circuit, and instinctively it feels like the wrong approach to take. Yet there is a fair amount of evidence on these forums of constant voltage circuits working. So why not use the same ICs we've been building into DDL drivers, and use them in their intended capacity as constant voltage sources? The benefit here being that we could actually begin to unlock the incredibly low ~0.3V dropout of newer LDO/ULDO drivers.
The result might finally be a DIY linear circuit that can run a RED from a single lithium ion, or 445 builds that continue to provide their output at lower levels of battery charge.
I was testing out the MIC29312 IC in a DDL-like configuration. Jufran88 was ambitious enough to put together a PCB based on my suggestion that it would be a good IC to test out. The benefit to this IC, in theory, being that it has an insanely low dropout of 0.35V at 2A current. I won't jump too far into my findings with that IC in the end (Although Jufran88 - your PCB design works great now! The setup just doesn't give us the low dropout we were hoping for). Rather, this thread is really about the fundamental limitation we're encountering in ALL implementations of DDL drivers.
Specifically, DDL drivers use voltage regulators like the LM317, LM1117, and more recently the 1084, 1085. Now, some even fancier ICs like the MIC29312 are starting to poke their heads up. However, the design of the circuit itself is the new limiting factor.
Why? Because our DDL circuit relies on the fact that these voltage regulators work by maintaining a drop of 1.25V across two of their pins. As we change the resistor value between those two pins, the regulator needs to pump more (or less) current across the resistor, in order to engage Ohm's law in a way that produces a 1.25V drop. By that process, the current moves (in an LM317 setup) out from the "out" pin, across the resistor, dropping 1.25V, at which point the drop is measured at the "Adj" pin, and the current then continues on to our laser diode. The fundamental limitation, is that this 1.25V drop must ALWAYS occur, and will (in a DDL circuit) do so in the path of current that flows to our diode. In other words, our DDL circuits will never drop LESS than 1.25V, even with an imaginary IC that could achieve 0V dropout.
So, what's the solution?
The solution I would like to toss out for discussion is not a perfect one. However, short of finding an IC with a lower-than-1.25V reference voltage, this may be out next best shot at improving the DIY linear driver space. For some time, users here have been building 405s, 445s, and even 650s with cheap LED drivers. Yet cheap LED drivers aren't typically constant current sources, they're generally constant voltage sources.
There are drawbacks to a constant voltage circuit, and instinctively it feels like the wrong approach to take. Yet there is a fair amount of evidence on these forums of constant voltage circuits working. So why not use the same ICs we've been building into DDL drivers, and use them in their intended capacity as constant voltage sources? The benefit here being that we could actually begin to unlock the incredibly low ~0.3V dropout of newer LDO/ULDO drivers.
The result might finally be a DIY linear circuit that can run a RED from a single lithium ion, or 445 builds that continue to provide their output at lower levels of battery charge.
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