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My application requires a constant current source for driving LEDs across a large voltage range (60-300VDC), and also the possibility to do this on both the high side and low side of the LEDs.

Microchip AN-H40 shows the LR8 linear voltage regulator being used as a constant current LED driver, much like similar arrangements for the LM317 or other linear regulators. In this configuration the device looks a lot like the Diodes AL5890, except with the addition of a capacitor.

Is it possible to use this configuration on both the high side and low side of LEDs, as shown in this circuit?

In theory the current drawn on both sides will on average be the same, but it had occurred to me that without C1 there would be larger ripples on the input side than the output side. Are the capacitors needed at all for a 1mA source?

circuit schematic

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  • \$\begingroup\$ You can always put the capacitor footprint on your pcb, and check if it is really needed later, when you test the circuit. One thing is for sure, the output capacitor C2 is needed, so that when LR8N8 starts, it does not oscillate. It can be unstable for the first milliseconds when it start, you may be able to see in using an oscilloscope (with/without C2) \$\endgroup\$
    – Christianidis Vasilis
    Commented Mar 15, 2021 at 0:09
  • \$\begingroup\$ I think the value of R1 might be in error. 1.2V/10k\$\Omega\$ is 0.12mA. The datasheet specifies 0.3mA typical minimum output current, and 0.5mA as maximum minimum output current. And sadly, at 450V (I know that is not your voltage, but...) the maximum output current is also 0.5mA. \$\endgroup\$
    – Math Keeps Me Busy
    Commented Mar 15, 2021 at 0:22
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    \$\begingroup\$ Linear regulators over such a large voltage range will get very hot \$\endgroup\$
    – D.A.S.
    Commented Mar 15, 2021 at 1:28

1 Answer 1

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The DC current at the through D1 will be the same as the DC current through D2. C2 is suggested by the manufacturer, and probably necessary for stability.

C1 may not be required. You might want to check that there is not a current spike through the input when 300V is suddenly applied (after a long time with no power and after short times). Also consider a resistor on the input (may affect stability). Neither capacitor has much voltage across it with single LEDs and under normal conditions.

Since you have 60V to play with you might also want to consider a series diode on the input eg. M7/1N4007.

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