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TPS798-Q1: About feedback resistor , output capacitor , Low-Voltage Tracking.

Part Number: TPS798-Q1
Other Parts Discussed in Thread: TPS797-Q1,

Hi Team,

My customer has a qustion.

Q1.

Datasheet P10 7.3.1 Adjustable Operation section has the following description.
"The adjustable device is tested and specified with the FB pin tied to the OUT pin and
a 1 mA - DC load (unless specified specified) for an output voltage of 1.275 V. "

At present, the output current is as low as 0.1 mA,
The FB resistance value is made small to flow 1 mA.

Does the power supply output have to be 1 mA or more?

I would like to increase the FB resistance as much as possible.
Example:High side resistance is 2.082 mohm Low side resistance: 220 kΩ

Also, when setting FB resistance high, how much should Feedforward Capacitor be set?

Q2.
Although the recommended value of the output capacitor is 1 μF or more,
Since the output current is quite low this time (Iout = 0.1 mA) this time,
Is it OK to change to 1 uF or less?
Please tell me the minimum value of Cout.

Q3
Answer to the following questions is not mandatory.

Datasheet P 12 7.4.1 Questions about Low-Voltage Tracking.
The Low-Voltage Tracking function,
How long (Delay) will it operate after the input voltage drops below the output voltage

Best Regards,

-watanabe

  • Hi Watanabe,

    A1) The source supply to the LDO IN pin must be able to supply enough current for the LDO load, the LDO GND pin current (this is the current used to power the LDO), and the feedback resistance current.  As your load current is 0.1 mA, Ignd is ~180 uA (max from the electrical characteristics table), and the feedback network is ~5.8 uA (typical: 1.275 V / 220 kΩ), your source supply must be able to supply at least 1 mA.  You will likely want the ability to supply more current in order to supply the inrush current to charge the output capacitor at startup.  Note that the LDO will only source what the load requires when it is in regulation.

    We generally recommend that the resistor divider current be ~100x the feedback pin current (Ifb = 0.2 uA for TPS797-Q1) for best accuracy.  By setting the feedback current this much higher, the feedback current will have a negligible impact on the resistor divider voltage.  As such we would recommend that R2 be no larger than 63.75 kΩ (1.275 V / 20 uA) for best accuracy.  larger resistors can be used; however, accuracy will decrease as Ifb becomes more significant in the resistor divider.

    The feedforward capacitor should be 100 pF and should be selected so that the impedance at 10 kHz is less than the resistance of R2.

    A2) We recommend that the output capacitor be 1 uF or greater for stability.  This is to compensate the internal feedback loop. 

    A3) The output will track the input as soon as it falls out of regulation which is when Vin falls below Vout + Vdropout.

    Very Respectfully,

    Ryan

  • Hi Ryan.

    I appreciate for your support.

    Q1.
    I am sorry that the question contents are hard to understand.

    Is it necessary to make Iout = 1 mA or more in order to operate the LDO stably?

    Currently, ILoad = 0.1 mA,
    By steadily flowing current to FB,
    Iout = 1 mA or more.

    Currently, ILoad = 0.1 mA,
    By passing current through the FeedBack line,
    Iout = 1 mA or more is secured.

    ILoad 0.1 mA + IFeedbackLine 1 mA = Iout 1.1 mA



    Best Regards,

    watanabe
  • Hi Watanabe,

    TPS798-Q1 does not have a minimum load requirement. Your load condition is okay and does not need to increase for stability.

    Very Respectfully,
    Ryan