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BQ25504: Minimum input values?

Mehmet Efilti1
Prodigy 30 points
Part Number: BQ25504

Hello

I would have a question about BQ25504 ultra low-power boost converter. In the data sheet of the device, the minimum starting voltage is 600mV but no information about the minimum input current. In the circuit I will design, the input current is at the microampere level. My input voltage value is maximum 600mv. What will be my output voltage and current in these input values? Also, can the battery be charged with these values? 

Is there any source I can test this device in the simulation environment?

Thanks in advance for your help.

  • 0
    TI__Guru*** 143880 points

    Hi Mehmet,

    In cold start, when VSTOR < VSTOR_CHGEN=1.9V, the cold start circuit typically operates down to 50mW of input power, which would be 15uW/600mV = 25uA of current.  The cold start circuit is a very inefficient boost converter with efficiency in the 10% range.  The output voltage is not regulated as the intent is get VSTOR up to 1.9V in order to exit cold start mode.  If you have a non-capacitive (resistive) load on VSTOR, I recommend adding a PFET with gate driven by VB_SEC_ON to isolate that load until the VSTOR voltage rises above VBAT_OK_HYST > VBAT_UV=2.0V max.

    I recommend using a power supply (or sourcemeter) with current limit (source) set to 25uA and voltage (compliance) set to 600mV for the input and a sourcemeter (like a Keithley) to simulate the battery.  See app note at the link below for an example:

    Regards,
    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Actually I do not understand what VSTOR voltage is. I guess the VSTOR value is the voltage we will get at the output. I do not understand whether the VSTOR voltage must be entered for the circuit to work or whether the output voltage of the circuit must come to VSTOR value. 

    When I examined the file you sent, I realized that there should be a voltage source to be connected to VSTOR, apart from the harvest circuit to be connected to VIN_DC. Did I get it right?

    I have one more question. When we think that we have only 600mV output voltage and approximately 30-100uA output current. This is a harvest circuit. Is it possible to operate the BQ25504 device and get efficiency with these input values without any other source? If not possible, what should I add? 

  • 0 in reply to Mehmet Efilti1
    TI__Guru*** 143880 points

    Hi Mehmet,

    VSTOR is the output of the boost converter.  It is pulls double duty as the power to a system load and power to IC itself once out of cold start. 

    The 4 quadrant (meaning capable of source and sink) voltage source at VSTOR (=VBAT if the UV protection PFET is on) is intended to simulated a storage element like a battery.  If you want to include the losses across the PFET, then I recommend placing that source at VBAT not VSTOR.  If you want an accurate current measurement, the source must be able to average its current measurement over a long period in order to account for the pulsing nature of the currents.

    With 600mV and 30-100uA, the cold start circuit will operate but it will take a long time to charge a real battery since the battery at VBAT is connected to VSTOR through the PFET diode.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Thank you very much, your answers are very understandable. But I have one last question for you. 

    When we think that we have only 600mV output voltage and approximately 30-100uA output current. What would be the approximate output voltage and output current we can achieve?

  • 0 in reply to Mehmet Efilti1
    TI__Guru*** 143880 points

    Hi Mehmet,

    In either cold start mode (VSTOR<1.9V) or regular boost mode (VSTOR>1.9V), the boost converter will be able to charge a capacitive load like a battery.  Because the charger operates in discontinuous, pulse frequency mode, it is difficult to compute the average input current (IINavg) and there use the efficiency equation below to predict output power:

    efficiency = VSTOR*ISTOR/(600mV*IINavg)

    You can use your 30-100uA as a estimate for IINavg and the datasheet efficiency curves (figs 1-7 starting on page 9) to predict efficiency in regular boost mode for different VSTOR voltages as the battery charges. In cold start, efficiency is in the 10% range.

    Regards,

    Jeff