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BQ25570: The buck output goes to zero for small loads

Vahid Montazeri63
Prodigy 230 points
Part Number: BQ25570

Hello,

I have the following circuit:


The solar cell open circuit voltage is a maximum 1.5V. These are the desired values:

VBAT_OV = 3.63V, VBAT_OK = 2.94V, VBAT_HYST = 3.34V, VOUT = 2.4V.

The capacitors are all electrolyte except the 10nF ones which are ceramic. The circuit is on a bread-board.

The VSTOR pin in the circuit is not connected to ground. It's connected to the other side of the capacitors.

I have four questions:

1-      The battery charges up to 3V but not beyond that. However, when I replace the solar cell with a DC supply at 1.5V, it continues to charge. What could cause the solar cell from charging the battery beyond 3V? Please note that the VOUT_EN pin was set to ground during the charging process. So there was no load on the buck converter. 

Also, I noticed that the VIN_DC voltage fluctuates a lot, even though the voltage of the solar cell is constant when it's not in the circuit. Why would this happen? I've noticed that higher CIN capacitors decrease the VIN_DC voltage but obviously make the voltage more stable.

2-      The bigger problem is the output voltage. For some reason it fluctuates a lot for different loads. When RL = 1KOhm (2.4V/1K = 2.4mA current), the output is zero for COUT = 47uf, but it is 2.4V for COUT less than that (20uf or lower). In this case VSTOR and VBAT were above 3V.

When I change the load to 220 Ohm (10.9mA current), the output goes to zero even for COUT = 20uf. I replaced the COUT with a ceramic 0.22uf capacitor and in this case the output (VOUT) and VSTOR, VBAT go to 2.4V. It seems that the circuit is not able to drive 10.9mA which is much lower than the datasheet values. What could cause this behavior?

3-      When RL = 1KOhm, and VOUT=2.4V, VSTOR=VBAT>3V, I measured the VRDIV pin voltage and its like this:


Is this normal? 

Regards,

Vahid Montazeri  

Update: I increased L2 to something higher (for example 330uH) and now the output voltage is much more stable for smaller loads. Nowhere in the datasheet has used such a high inductor so I'm a little bit concerned on why my circuit needs such high inductor.