Cant explain current drop in TPS61220

If the output voltage is in regulation, then the output current is a function of your load resistor.  If the output voltage is dropping out of regulation, then your solar panels are probably not capable of your required power.  Be sure to do some research to understand the load line of a solar panel.  A 300mA, 3.5V solar panel is not capable of 300mA AND 3.5V at the same time.  This is probably an issue that can be solved quickly with assistance from your local FAE.  If you don't have that contact info, you can get it by contacting TI's Product Information Center  http://www-k.ext.ti.com/sc/technical-support/product-information-centers.htm?DCMP=TIHomeTracking&HQS=Other+OT+home_d_contact 

Thanks for the reply Michael.  Just to clarify :

Solar panel voltage and current as mentioned in the post ( 3.5v & 300ma) is based on panels in full illumination with a 100ohms load resistor. When I connect the solar panel output to the input of the boost module, I do get the desired 5V regulated voltage but the effective current is only 70ma whereas I would be expecting at least 200ma from the TPS61220 since the source is already delivering the same.

I will give the FAE a call and see what they have to offer, and hopefully some useful insights can be got from here too.

Thank you again and regards

It is important for you to remeasure your voltages and currents.  I do not understand how you can have 3.5V across a 100 ohm resistor and only get 300mA through it.  The equation V=IR must be valid.  Also be aware that if you have a 3.5V @ 300ma source which is 1.05V and get a 5V @ 200mA = 1.00W, it requires that the power conversion be 95.2% efficient.  This will not be possible with the TPS61220.  The power capability of the IC is given in Figure 1 of the datasheet.  Also be sure to measure the output voltage of your solar cell with an oscilloscope to make sure you know the actual output voltage.  I think you will need to consider using the TPS61200 for your application. The TPS61200 has a good solar charging application note to help you maintain maixmum power from your solar panel

Dear Michael Thank you for the response. Im sorry I mistook another project measurement with this.

Yes, the voltage/current measurement is 3.5v-4.25/300ma (peak illumination across 3 panels connected in parallel). Each panel is rated 3.5v-4.25/100ma and the values were a short-circuit measurement.

I have 6 panels supplying a total of 600ma if connected in parallel.

While reading through other forum posts I came across another posting dealing with a charging application using the suggested  TPS61200 chip. My primary application is to use the solar panels as a charging outlet for phones or NI-CD/Ni-MH batteries. In short, whereas the batteries would require a dedicated charging IC and associated circuitry, I would like a full USB-like output with 5V and 500ma output for phone charging. 

What kind of design considerations am I looking at for this ? Can the panels provide such an output directly (via a boost/switching circuit) ?

Thank you and regards 

Your panels should be able to support the DC power required, in addition to the losses that occur during the conversion.  It would be best to oversize your panels a little bit to give some margin in the output power required and to be able to supply the power required for step load changes.

The TPS61200 is a good IC to look at, with its app note on using solar cells.

Thank you Chris, I am going to give this chip a try soon.