Hello all. I hope someone can help me out with a challenge I'm facing.
I'm trying to build an LED driver based around the Diodes ZXLD1360 driver IC. The circuit works, as far as powering the LED up from a wide range of voltages, but I'm having a little difficulty with the dimming interface.
Here are my requirements:
A PNP transistor (2n2907, currently set up on a breadboard) between the ADJ pin and ground gives me the logic that I want, and with a pull-up resistor between emitter and base I get the response I want when the dimming signal is removed, to a degree. With this setup, the pull-up resistor is altering the voltage at the ADJ pin beyond my desired spec. With 1K on the base and 10K as the pull-up (collector ties directly to ground), pwm voltage is about 3.25v. When the input is pulled, the voltage drops to 1.44v, which is acceptable, but this gives half the output compared to when the input signal is at 100% duty cycle.
A suggestion I was given was to add an N-channel FET (2n7000, again, on a breadboard) in front of the PNP, with the base of the PNP tied to the drain of the FET. Source is tied to ground through a 100K resistor, gate has a 100K pull-down resistor, and the gate to the micro goes through a 1K. With the FET in place, the voltage at the ADJ pin comes down to about 2.5v peak. When the input signal is pulled though, the output shuts off, and the whole circuit has about 400mV across it, effectively pulling the ADJ pin into a low state.
One thing I had thought about doing was to keep the PNP setup, and add a 2.5v zener (with matching series resistor of course) to the incoming pwm signal as a voltage shunt. This should cap the voltage at the base of the PNP to more manageable levels, and should be voltage agnostic as far as the incoming signal is concerned. I went with 2.5v because anything less than that gets expensive really fast (a 1.25v shunt regulator would cost almost double what the entire driver BOM costs at the moment). A point of concern though was the high leakage current (seems to be common at this low of a voltage). I wasn't sure if 100uA was going to cause any problems. I have some sample parts on the way for that to test it out.
Anyway, that's what I've tried so far, and what I've found as a result. Can anyone suggest any better alternatives? My electron-fu isn't that great, but I learn fast.
Thanks in advance.
I'm trying to build an LED driver based around the Diodes ZXLD1360 driver IC. The circuit works, as far as powering the LED up from a wide range of voltages, but I'm having a little difficulty with the dimming interface.
Here are my requirements:
- LED brightness must follow incoming pwm duty cycle (i.e. 0% duty cycle = 0% output, 100% duty cycle = 100% output)
- Output must default to 100% output when dimming signal is removed (floating)
- Must be compatible with 3.3v and 5v micros
- Must be small, as I'm trying to keep the pcb to 13mmx13mm. I know that's tough, and will expand the board size if necessary, but I have a little room to work with.
A PNP transistor (2n2907, currently set up on a breadboard) between the ADJ pin and ground gives me the logic that I want, and with a pull-up resistor between emitter and base I get the response I want when the dimming signal is removed, to a degree. With this setup, the pull-up resistor is altering the voltage at the ADJ pin beyond my desired spec. With 1K on the base and 10K as the pull-up (collector ties directly to ground), pwm voltage is about 3.25v. When the input is pulled, the voltage drops to 1.44v, which is acceptable, but this gives half the output compared to when the input signal is at 100% duty cycle.
A suggestion I was given was to add an N-channel FET (2n7000, again, on a breadboard) in front of the PNP, with the base of the PNP tied to the drain of the FET. Source is tied to ground through a 100K resistor, gate has a 100K pull-down resistor, and the gate to the micro goes through a 1K. With the FET in place, the voltage at the ADJ pin comes down to about 2.5v peak. When the input signal is pulled though, the output shuts off, and the whole circuit has about 400mV across it, effectively pulling the ADJ pin into a low state.
One thing I had thought about doing was to keep the PNP setup, and add a 2.5v zener (with matching series resistor of course) to the incoming pwm signal as a voltage shunt. This should cap the voltage at the base of the PNP to more manageable levels, and should be voltage agnostic as far as the incoming signal is concerned. I went with 2.5v because anything less than that gets expensive really fast (a 1.25v shunt regulator would cost almost double what the entire driver BOM costs at the moment). A point of concern though was the high leakage current (seems to be common at this low of a voltage). I wasn't sure if 100uA was going to cause any problems. I have some sample parts on the way for that to test it out.
Anyway, that's what I've tried so far, and what I've found as a result. Can anyone suggest any better alternatives? My electron-fu isn't that great, but I learn fast.
Thanks in advance.