BQ25570： About one way to avoid cold start?

Hi Yujian,

The IC runs in cold start if VSTOR < VSTOR_CHGEN=1.8V typical. If you attach a partially charged battery to VBAT > 1.8V, the charger does not enter cold start. If the problem is a load on VSTOR that is preventing you from exiting cold start, then you can add a PFET from VSTOR (as mentioned above for BQ25505) to your load, driven by VBAT_OK that is inverted by an NFET.

Regards,
Jeff

Hi Jeff，

Thank you for your reply.

About  add a PFET from VSTOR to my load.I know  a way for BQ25505:The drain of the first PFET should be connected to VSTOR and the drain of the second PFET should be connected to the load. The source of both PFET are connected together. The signal /VB_SEC_ON should be connected to the gate as shown in the datasheet.The above is from Solar Application Circuit with BQ25505.

Question 1，you know, my IC is BQ25570. I want to know if the PMOS Thing also applies to BQ22570(for this ,BQ25505=Bq25570,???).

Question 2, my problem is that I can't connect the NFET to VBAT_OK correctly.

This is my circuit:

best,

deng

Hi jeff,

Thank you again for your reply.

I seriously read your answer.Then i modified my circuit diagram.as shown in illustration.

for  two power rails:

for one power rail:

There is one thing bad.I don't know if have two power rails.I just want to avoid cold start or making cold start much faster.Because  sufficient input power  is not available, it is possible that the cold start circuit cannot raise the VSTOR voltage above VSTOR_CHGEN in order for the main boost conveter to start up.

I will experiment with  the two circuit.  Do you have any suggestions for me?

Thanks again

best,

deng

Deng,

To speed up cold start circuit charging up VSTOR and VBAT (through the internal PFET body diode), you need to remove any other loading on VSTOR by tying VOUT_EN to VSTOR or VBAT_OK and using the external PFET, driven by VBAT_OK and NFET, to isolate any other load on VSTOR. If the storage element at VBAT is discharged below VBAT_UV = 2.2V, then cold start will take a long time to charge it up. Alternatively, you could connect a partially charged battery at VBAT > 2.2V and the charger will avoid cold start.

If you do not need the buck converter for a second power rail, you could place your storage element (battery or super capacitor) at VOUT and only place a 100uF capacitor at VBAT. VBAT_OV needs to set at least 300mV higher than VOUT_SET. With VOUT_EN=VSTOR, VBAT=100uF will charge quickly in cold start, enter main boost charge and then turn on the buck converter. The buck converter will then turn on briefly but turn off as its input droops below its input uvlo. The input uvlo is higher than the boost charger's VBAT_UV so the boost charger stays in main boost and the buck converter turns on/off to charge the storage element at VOUT. This will be faster than using cold start.

Just to confirm, your input source voltage is higher 600mV and can provide at least 50nA a current?

By the way, you should not connect VBAT or isolate VSTOR from PFET directly to VOUT. The buck converter input is already connected internally to VSTOR.

Regards,
Jeff

Hi Jeff,

My input source voltage is lower 600mv and can't provide 50nA current.

best,

deng

Hi,Jeff 

I have to ask you help again.

when i add a PFET and a NFET to my circuit,I want to know the IC whether can avoid cold start and enter main boost charge  in VINdc<600mv.

My input source voltage is 200mv-300mv. 

Can  my circuit work?

best,

deng