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BQ25505: Very dirty output power

Henrique Reis
Prodigy 230 points
Part Number: BQ25505

Having an issue with this.

Input battery power is set to 3.2V, the circuit beyond the bq25505 draws about 100 to 200mA (I can't find anything that says what the maximum output current of the bq25505 is)

Solar panel delivers about 5V (1Watt) panel.

There is a supercap to charge  on vbatt_sec

The resistor setup is setup as below.....hope you can see this.

What we are running into a high frequency oscillation when vstore rises above 1.8V.  It then cleans up when vstor gets to about 3.3V

VBAT_UV < VBAT_OV < 5.5V VBAT_OV > VBAT_OK_HYST > VBAT_UV 1.3V < VOUT < VBAT_OV MPPT
Desired           RSUM1 13 MW               RSUM1 20 MW    
Desired RSUM1 13 MW     VBAT_OK 1.8 V > VBAT_UV   RSUM1 13 MW     VIN_DC(OC) 4 V Open Circuit Volts  
Desired VBAT_OV 3.8 V     VBAT_OK_HYST 2 V > VBAT_OK   VOUT 1.3 V     VREF_SAMP 3.2 V MPP voltage  
                                       
              closest 1% resistor1                 closest 1% resistor1  
    closest 1% resistor1     Exact < >        closest 1% resistor1     Exact < >   
Computed   Exact < >    ROK1 7.865 7.680 7.870 MW   Exact < >    ROC1 6.000 5.900 6.040 MW
Computed ROV1 6.209 6.190 6.340 MW ROK2 3.835 3.830 3.920 MW ROUT1 2.100 2.050 2.100 MW +10MEG2 10.000 10.000 10.000 MW
Computed ROV2 6.791 6.650 6.810 MW ROK3 1.300 1.270 1.300 MW +10MEG2 10.000 10.000 10.000 MW ROC2 4.000 3.920 4.020 MW
Computed VBAT_OV
 
3.765 3.765 V VBAT_OK
 
1.813 1.813 V ROUT2 0.900 0.887 0.909 MW +10MEG2 0.000 0.000 0.000 MW
Computed           VBAT_OK_HYST   2.014 2.013 V VOUT
 
1.299 1.301 V VREF SAMP
 
3.209 3.198 V
                                       
Selected ROV1 6.34 MW     ROK1 7.87 MW     ROUT1 2.1 MW     ROC1 6.04 MW    
Selected ROV2 6.81 MW     ROK2 3.92 MW     '+10MEG2 10.00 MW     +10MEG2 10.000 MW    
Selected  
 
      ROK3 1.3 MW     ROUT2 0.9 MW     ROC2 4.02 MW    
           
 
       
 
      +10MEG2 0.000 MW    
Typ voltage3 VBAT_OV(typ) 3.765 V -0.94 % diff VBAT_OK (typ) 1.813 V 0.70 % diff VOUT(typ) 1.300 0.00 % diff    
 
     
Typ voltage3           VBAT_OK_HYST (typ) 2.013 V 0.62 % diff           VREF_SAMP 3.198 V -0.05 % diff
         
  • 0
    TI__Guru**** 166976 points
    Henrique,

    Can you elaborate on what you mean by dirty output power?

    To answer a few of your questions, output power can be estimated using an efficiency balance as eff=POUT/PIN = VSTOR*(ISTOR+IBAT)/(VMPP*IINAVG). IINAVG is difficult to compute because of the boost charger's DCM operation. But, if your power source is low enough impedance (i.e. oversized) then the boost converter will run at maximum input current (200mA ILpk) at the edge of CCM and DCM without pulse skipping which gives IINAVG = 100mA, then it is easier to compute.

    The max switching pulse frequency is 1MHz, which you will only see if you are running the low impedance/over sized input source configuration mentioned above. Due to the DCM pulse frequency operation, the switching operation for most higher impedance power sources will be pulses and then pauses, similar to d/s figures 18, 30 and 36.