LP55231: I need to drive one of the LEDs with an external signal as well. Is there any problem with this?

David Burton

Part Number: LP55231
Other Parts Discussed in Thread: LP55281, LP5569

I would like to also be able to drive one of the LEDs with an external signal. I'm thinking I can just use a high side switch and series R from the Vout pin to the LED anode I wish to drive. Seems from the data sheet the outputs can be overdriven up to the supply voltage + .3V, though Vout could potentially be high than that?

The outputs shows as DAC controlled current source between Vout out and the LED drive pin.

Sketch showing idea attached. Can anyone comment on if this is OK or not?

Thanks.

over 7 years ago

0 David Burton over 7 years ago

Expert 1330 points

Maybe a schottky diode on the output of D1 to protect it from Vout is needed?

0 Rosley Ge over 7 years ago in reply to David Burton

TI__Intellectual 950 points

David,
I don't think this circuit can work. The driver output is controlled by internal FET. It will connected to supply based on the command. If you add something like your drawing. I think the internal FET can't work normally.
Thanks,
BRs,
Rosley

0 David Burton over 7 years ago in reply to Rosley Ge

Expert 1330 points

I would think with the added diode it would work so long as the driver is able to drive into the forward voltage drop of the diode + LED.

0 David Burton over 7 years ago in reply to David Burton

Expert 1330 points

0 Rosley Ge over 7 years ago in reply to David Burton

TI__Intellectual 950 points

David,
I don't know why you need to use this circuit. Then you should apply higher Vout, like 6V. In that case, you should consider the appropriate voltage of FET and when FET is turned on, 6V will be added to LED directly and the LED current is not controlled by LED driver.
Anyways, I think the circuit is not stable. I don't suggest it.
Thanks,
BRs,
Rosley

0 David Burton over 7 years ago in reply to Rosley Ge

Expert 1330 points

I need it because that particular LED needs to be able to be turned on by an external signal separate from the LED controller. When that occurs the LED controller would not be trying to turn on the LED. If it did the LED current would be increased. I think the Vmax rating on the output is Vdd+.3V and Vload could be at 4-5V, but the IR drop though the current limiting resistor and MOSFET as clamped by the LED Vf should protect from that.

We are trying to replicate the function of this circuit as implemented using a low side LED driver (LP55281), which is easier, but adaptation is need for this driver being high side.

A more straight forward solution would be to use a low side driver like the 281, but in a larger package since the design rules of the board this is going on do not support routing of that tiny BGA package.

Thanks.

0 Rosley Ge over 7 years ago in reply to David Burton

TI__Intellectual 950 points

David,
I suggest LP5569 to you. 9CH low side LED driver with QFN package. I think it may be better for your design.
Thanks,
BRs,
Rosley

0 David Burton over 7 years ago in reply to Rosley Ge

Expert 1330 points

Yes that looks like a better option. Thanks for pointing that one out.

Power management

Power management forum

LP55231: I need to drive one of the LEDs with an external signal as well. Is there any problem with this?