LM3553SDEV: If the input voltage is 5.5V,output voltage will still be 5V?

Hi,

Considering of Vf of diode(~0.7V) and efficiency lost, if LM3553 operates in voltage mode, it may wok but I can't guarantee this.

Robin

Hi Yuting:

If input voltage is 5V,the voltage will bypass.

So the output voltage is 5V.

Input under 5V, the output will boost to 5V.

Input above 5V, the output can not be guaranteed?

Is this description right?

best regards

Hi,

Yes. Why not considering of buck led driver? e.g. TPS92200.

Robin

Hi Yuting:

For new project,I have change to TPS92200.

But the lead time is 50 weeks.

And there is no stock in TI inventory.

Digikey mouser have zero in the stock.

I am considering to change to other company product.

I only need 1K for this year.

But there is really no stock.

Design is one thing, mass production is another.

best regards

Hi Yuting:

I know LM3405 have many in stock.

But our product is optical device.

I can only change the current of LED.

I can not use PWM to on off LED.

So I choose TPS92200,using analog signal to change the current of LED.

But LM3405 can only accept PWM dimming.

LM3405 may not be suitable for this project.

best regards

Hi,

What's your concern about PWM dimming? In fact, LM3405 can achieve analog dimming with some help of external circuit.

Robin

Hi Yuting:

Because our project has LED and photodiode.

The photosensor must synchronize the LED.

So we need LED to continuously lighting during the sensing period.

For LM3405, I can not see external analog dimming circuit in the datasheet.

Please help to teach me.

I can provide 0~3.3V analog signal to control brightness.

I hope the analog dimming circuit is easy to use.

best regards

Hi,

Sure, I can help you about how to configure it. But there is still something that makes me confused: I have encountered similar applications utilizing sensor signal to control LED on or off, normally, the LED PWM frequency is higher than sensor signal frequency e.g. 30Hz is enough. They don't need analog dimming. What's your comment about this?

Robin

Hi Yuting:

Our sensor signal sampling frequency is about a few K samples per second.

And the firmware engineer do not want to use PWM to programming.

The synchronizing frequency is a little high.

Maybe this is a difficult point.

best regards

Hi,

OK. I'll let you know the analog dimming method later.

Robin

Hi Yuting 

I wish to hear good news for analog dimming method.

Best regards 

Hi,

Please refer the section 9.22 of TPS61165 datasheet.

Robin

Hi Yuting:

Do you mean Figure16 of page 18 of TPS61165 datasheet ?

I only see PWM signal on FB pin.

I still do not understand analog dimming method of LM3405.

Could you help to explain it?

Thank you

best regards

Hi Yuting:

I missunderstand.

I can apply a analog signal 0~3.3 to control the lighting.

But we have difficulty applying a PWM signal.

Maybe we need to wait for the TPS92200D1DDCR stock ?

Mouser have apply a order to TPS92200D1DDCR.

The ship date is 9/14/2022.

So we need to wait this small chip for one year.

Or you can advise other parts to replace TPS92200D1DDCR?

We have only one LED to drive.

Vf is 3V to 8V. Current is below 350mA.

Thank you

best regards