BQ25570 EVM Simplest Hookup

The buck will not start operating until VSTOR >VBAT_UV =1.95V typical.

Assuming you are connecting the solar cell to the bq25570EVM-206 and the storage element on VBAT

JP2 - /EN=GND

JP4 - VOC_SAMP=80%

For faster charging of a super cap, you can place it on VOUT instead of VBAT and ensure JP3 sets VOUT_EN=VSTOR.  Since the buck will isolate the supercap from the boost charger until VSTOR >VBAT_UV>VSTOR_CHGEN, the cold start circuit will more quickly charge VSTOR >VSTOR_CHGEN since it is not loaded by the supercap.  Then, the main boost charger will provide power to the buck converter for charging the supercap. 

Thanks for the super fast response.

Does the buck converter regulate the output appropriate even if the supercap is not connected to the VBAT, VSTOR just needs to be above VBAT_UV for it to work, correct?

VSTOR just needs to be above VBAT_UV for the buck to work.  If you have large load transients on VOUT, you might need a slightly larger capacitor than the 4.7uF capacitor that is on the EVM (assuming you removed the 100uF on VBAT).

So the customer is still having some issues with getting the correct output at Vout (J11)

Solar cell Voc = 2.5 @ J2

JP2 is jumped to GND

JP4 jumped to 80%

JP3 jumped to VOUT_EN = VSTOR

JP5 jumped to VREF_SAMP = GND

The emasured output at J11 is 0.3V. Is the problem because of the JP3 and JP5...If so, why would those cause this problem?

Remove JP5.

What is the resistive loading on VSTOR (including any leakage current of the storage element attached to VSTOR)?  What is the resistive loading on VOUT?

If the loading is too high, VSTOR will never rise to 1.8V and the IC will not leave cold start.  Even if the IC leaves cold start, the boost converter may not be getting enough power for the load on VSTOR + the buck converter.  The following power balance equations will help:

PIN-buck > POUTbuck/buck_efficiency

PIN-boost>(POUT-vstor + PIN-buck)/boost_efficiency

PIN-boost = MPP per solar panel datasheet.

You can get estimated efficiency values from the bq25570 datasheet.

Also, as the datasheet and EVM user's guide warn, if you change the >1Meg resistors on the EVM, you must remove all of the solder flux from between/across the resistors.  Solder flux residue has resistivity in the Megaohm range, which means the resulting parasitic resistors will change the effective resistances of your resistors and change your regulation points.

Ok, thanks...from what i know there is nothing hooked up on the output, the customer is just measuring straight from the Vout/Gnd terminals

There is also nothing connected to the VBAT terminal (no storage element connected)

Response from the customer:

I have remove JP5 jumper, but still have ~0V at J11

Yes, there are nothing attached to VSTOR nor VOUT

The solar cell that I used has Voc =2.4V, and Isc=10uA. When I connect the solar cell to J2, the voltage drops down to 0.3V.

Voc=2.4v – 3V, Isc=10uA is the typical condition of the solar cell will be used.  Thus, would you please confirm if the IC can work under such condition?

Yes.  See datasheet figure 7. 

When you take JP3 to GND instead of VSTOR, to what voltage does VSTOR rise?

The following is the test result with different jumper setting

Solar cell output : Voc  = 2.384V,  Isc = 10uA

No connection at J5, J8 and J11

JP5 must be OPEN and JP2 must be GND for the IC to boost.  If Vin is only 0.345, then the solar cell isn't getting enough light and so can't provide enough current to get the IC out of cold start.  Can you increase the light source?  Alternatively, if you attach a partially charged battery to VBAT (assuming VSTOR=VBAT=0V before the attach), then the IC will work because VSTOR will be at > 1.8V.   To confirm that there is nothing wrong with the EVM, connect VIN_DC to a regular bench power supply through a ~1kohm resistor. 

I have done the test again with higher light intensity, the result shown as below table

J2 GND, J3 GND, J4 80%, JP5 open

By J2, I assume you mean JP2.

By J3, I assume you mean JP3.  If so, JP3 should be tied to VOUT_EN to enable the buck converter.

What was the voltage on VSTOR?  If above 1.8V, then you should see a voltage on VOUT once JP3=VSTOR.

I repeat the test with the same jumper setting, and the result shows as below.

If JP3 is set to VOUT_EN and the resistive load on VOUT is not too small, there should be a voltage on VOUT. 

May I ask what  resistance range that I should put between Vout and the Supercap?

The bq25570EVM-206 user's guide (SLUUAA7) figures 11 and 12 shows how to charge a supercap on VOUT.  The supercap connects from VOUT to GND.  VSTOR is tied to VBAT.  There is no resistance load connected.

When in cold start, the input voltage is clamped to the VIN_CS value (~.350V).  Since the IC needs at least PIN(CS)=~5uW to exit cold start, IIN ~= 5uW / 0.350V =14uA is required to exit cold start, assuming no resistance load (including battery and capacitor leakages) on VSTOR or VBAT.   Higher lux on the panel gives more current and therefore you can exit cold start. 

With a discharged super cap, I recommend that you bypass cold start using diodes and FETs.  For example, since your VOC is higher than the IC's VSTOR_CHGN (~1.8V) to exit cold start, I recommend a schottky diode from the panel output to the supercap.  Since your solar panel may be power limited, I also recommend a PFET in between the panel output and VIN-DC, source tied to panel and gate tied to VBAT_OK set to 1.9V.  This is will shorten your charge time significantly.