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LP87702-Q1: What is the COUT_BUC K_POL?

Part Number: LP87702-Q1

Dear team,

Could you please hep tell me what is the COUT_BUC K_POL? What is this capacitor used for? Why does this capacitor exist?

Thanks & Best Regards,

Sherry

  • Hello,

    Please refer to section 8.2.2.1.3, Buck Output Capacitor Selection, of the datasheet, www.ti.com/.../lp87702-q1.pdf

    "POL capacitors can be used to improve load transient performance and to decrease the ripple voltage. A higher output capacitance improves the load step behavior and reduces the output voltage ripple as well as decreases the PFM switching frequency. Note that the output capacitor may be the limiting factor in the output voltage ramp, especially for very large (100-μF range) output capacitors. For large output capacitors, the output voltage might be slower than the programmed ramp rate at voltage transitions, because of the higher energy stored on the output capacitance. Also at start-up, the time required to charge the output capacitor to target value might be longer. At shutdown the output voltage is discharged to 0.6 V level using forced-PWM operation. This can increase the input voltage if the load current is small and the output capacitor is large compared to input capacitor. Below 0.6 V level the output capacitor is discharged by the internal discharge resistor and with large capacitor more time is required to settle VOUT down as a consequence of the increased time constant."

    Regards,

    Chris

  • Hi Chris,

    Thanks for your reply!

    Can I ask one more question? Currently my customer use LP87702KRHBRQ1 version in their design. I want to know whether they can change register value via I2C, or our device has fixed registers which customer is not allowed to change.

    Thanks & Best Regards

    Sherry

  • Hello,

        Yes, the customer can change the register values via I2C.  When the device is power cycled, then the registers will return to the default values defined by the LP87702KRHBRQ1 version. 

    Regards,

    Chris

  • Hi Chris,

    I understand POL capacitor's function now, but I don't know what the difference between Cout-buck and Cpol-buck is. In the layout, Cout-buck should be close to our chip pin while Cpol-buck should be close to the load, right?

    Thanks & Best Regards,

    Sherry

  • Sherry, 

        Yes.  That is correct.  Please note that there are examples in the product page under Hardware Development.  These are various EVMs which use the PMIC.  Also there is a layout example in figure 109 of the datasheet.


    Regards,
    Chris