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BQ24297: VREGN output clarification

Stefano Infante
Expert 2310 points
Part Number: BQ24297
Other Parts Discussed in Thread: OPA2990

Hello!

The BQ24297 data sheet states that the VREGN LDO output is 4.8 V (typical) when VBUS is 5V (Section 8.5 Electrocal Characteristics, subsection REGN LDO). But on the very next line, REGN LDO current limit, the "test conditions" are stated as "VBUS = 5V, VREGN = 3.8V". Is it a typo? Or does VREGN vary according to parameters such as output current?

What range do I have to expect from VREGN? I have an NTC connected to it in a voltage divider configuration (see Figure 17 in the data sheet), and I also need to read the NTC value (I have it connected to an ADC through a unity gain opamp), so I need to know if I can assume VREGN to be constant.

Finally, can I use VREGN to power up the opamp (OPA2990)?

Thanks in advance.

Stefano Infante

  • 0
    TI__Guru 50541 points

    Stefano,

    The REGN LDO current limit spec provides the minimum current limit is 50mA. When the REGN LDO is under current limit, the output voltage level could drop and 3.8V is just a test condition. It is not normal REN LDO output without current limiting.

    The REGN LDO supports internal circuitry. Please do not connect to any external loads besides the REGN connections shown on Figure 41 on the d/s.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Expert 2310 points

    Hi Ning,

    thanks for your reply.

    I just need a further clarification: referring to Figure 41, I use 18.2kohm in parallel with my NTC, and 4.32kohm as "upper resistor", so in the worst case (R_NTC -> 0) the maximum VREGN current would be 4.8V/4.32kohm = 1.1 mA. And I suppose internal circuitry is designed to draw from VREGN less that 50 mA. So, if I don't connect anything to VREGN, is there a chance that REGN current goes over 50 mA and limits VREGN output? In other words, can I assume VREGN to be stable at about 4.8V in normal work conditions?

    Thanks again,

    Stefano

  • 0 in reply to Stefano Infante
    TI__Guru 50541 points

    Stefano,

    As long as customized designs follow the Typical Application in Figure 41 on the d/s and the BQ24297 EVM design, the REGN LDO should work normally.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Expert 2310 points

    Hi Ning,

    I made some measurements, and found VREGN to be about 4.83V in most cases. But a couple of times I found it to be about 5V; is it an expected behaviour?

    Thanks,

    Stefano

  • +1 in reply to Stefano Infante
    TI__Guru 50541 points

    Stefano,

    Yes. The test results are normal. Please refer to the REGN LDO specs on the d/s.

    Thanks,

    Ning.