Electronics > Beginners
LEDs acting as photo cell, preventing SMPS startup
(1/3) > >>
avector:
I'm building an outdoor display using LEDs. Indoors, the display starts up fine, but outdoors it does not. I'm using a TI TPS63071 SMPS chip to drive WS2813b neopixels, and have implemented the TPS63071 example circuit (9.3 Typical Application for Fixed Voltage Version) in the datasheet without modification.

In my investigating, the LEDs are dropping Vout to about -0.3 or -0.4 V when exposed to the sun, before applying power. After powering up, I can touch the ground to Vout quickly and the display starts up. The datasheet for the TPS63071 shows the absolute minimum voltage for the Vout pin to be -0.3V, so I believe this to be the culprit.

My thought to solving this would be to add a transistor between the TPS63071 Vout and the LEDs that would be switched on when the TPS63071 PowerGood pin was high. I'm not certain the transistor would keep the voltage greater than or equal to ground though, and testing this solution will require me to spin a new PCB. Any thoughts about how to keep Vout above or equal to ground, or any problems I may have missed would be appreciated.

Datasheet
http://www.ti.com/lit/ds/symlink/tps63070.pdf
wraper:
You could simply add resistor between Vout and GND. Current from LEDs should be tiny. BTW what is resistance of feedback divider? I would expect it being low enough to sink current from LEDs.
EDIT: I checked datasheet and suggested resistor values are quite high. You could simply decrease them to ensure enough of current sinking.
thinkfat:
TI usually specifies a voltage divider with very high resistor values for devices with ultra-low quiescent current. If you specify a typical IQ of 2 µA into Vout, that makes sense. But if you don't care for that parameter you can use a lower resistor voltage divider which at the same time makes the feedback input less susceptible to noise from the inductor.
avector:
Oops, it seems I slightly goofed the part number, but I have the right datasheet linked. I should be TPS630701 instead of TPS63071. TPS630701 has Vout tied directly to the feedback pin, and the voltage divider is internal to the IC.

Per Wrapper's idea, a 100 Ohm resistor between Vout and GND did allow the display to start up, but a 470 Ohm did not. I measured 1.25 mA coming from the LEDs in sunlight. If I treat them as a constant current source, I would need at most 200 Ohms, preferably a bit less to ensure stability, to keep the voltage difference below 0.3V. Since Vout is a 5V when running, this will use about 0.125 to 0.25 Watts depending on the resistance chosen. I was hoping for a more power efficient solution since the display is running on batteries.
wraper:
I would guess then this is somehow related to negative voltage going into feedback pin. I would try either using TPS63070 which has voltage divider or TPS630702 which also has output discharge.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod