TPS61200 LED driver

Hi Chris

Were do I conect R3 since there is no external power supply ?

You don't need R2 or R3.  Just short where R2 was and open where R3 was.

Chris

Will that regulate 120mA for the LED, or I have to do some R1 calculations ?

Thanks

You will still need to calculate R1.  The equations simplify (without R2 and R3) to R1 = Vfb/Iled.  The IC is regulating the voltage at the FB pin.  This voltage is the LED current across the R1 resistance.  V=IR

Chris

What value dos Vfb have to be ?

I just measured and Vled is 3.06V, Iled is 156mA, am I am using a 3.3Ohms as R1

Vfb is the feedback voltage which is regulated to 0.5V.  This is in the datasheet.  That roughly lines up with your measurement.

Hi Chris

I changed R1 to 5 Ohms and was able to get 100mA, but at 4V Vin power consumption is about 117mA, is there a way to get this down ?

Thanks

No.  At 4V in and a single LED out, you are in down mode.  This is a special mode in some of our boost converters that keeps the output voltage regulated.  Most converters cannot regulate the output voltage if Vin > Vout.  The TPS61200 can through down mode.  In down mode, your input current will be slightly higher than the output current.

What is your Vin range?  With 0.8V in minimum, surely you don't need to operate at 4V in also?

Hi Chris

This part is going to be fed by a supercap which when is charged its voltage is = 4.50V and when discharged it could be 0V , but since the parts turns OFF at 0.8V I will limit that.

The idea is not to wast any current if it is possible (energy) .

Thanks

The TPS61200 runs down to a 0.3V input.

The other option is to use a TPS63xxx device, such as the TPS63000.  There is an EVM available that configures it as an LED driver.  The upside is that you would get better efficiency at high Vins, but the downside is that it cannot run to Vins as low as 0.8V.