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TPS60402-Q1: Iout vs Cfly and Rswitch information

Part Number: TPS60402-Q1
Other Parts Discussed in Thread: TPS60400-Q1, TPS60403-Q1,

Hello,

My customer looks at the datasheet and asked a couple of questions.  Would you please give me your comments?

1. Iout vs Cfly

My customer looks at 9.2.2,, but it doesn't say how Iout is related to the Cfly.  Would you please tell me how it is?

Note that they'll have Vin =5V and Vout=-5V with Iout=27mA(max) by Cfly=22uF, but they are't sure if they can.

9.2.2 Detailed Design Procedure
For the maximum output current and best performance, three ceramic capacitors of 1 μF (TPS60400-Q1,
TPS60403-Q1) are recommended. For lower currents or higher allowed output voltage ripple, other capacitors
can also be used. TI recommends the output capacitors has a minimum value of 1 μF. With flying capacitors
lower than 1 μF, the maximum output power will decrease.

2. Rswitch

They look at 8.3.1, but there is no information about Rswitch.  Would you please tell me about it?

8.3.1 Charge-Pump Output Resistance
The TPS6040x-Q1 devices are not voltage regulators. The output source resistance of the charge pumps is
approximately 15 Ω at room temperature (with VI = 5 V), and VO approaches –5 V when lightly loaded. VO will
droop toward GND as load current increases as seen in Equation 1.
(1)
where
• RO = output resistance of the converter
• RSWITCH = resistance of a single MOSFET-switch inside the converter
• fOSC = oscillator frequency

Best Regards,

Yoshikazu Kawasaki

  • Hi Yoshikazu,

    There is no max value for Cfly so you can use other values but I would recommend to use the value suggested in datasheet. More information for Rswitch can be evaluated through figure 9.

    Best regards,
    Tanvee
  • Hello Tanvee,

    Thank you for your quick reply.  Please let me confirm what you said.

    > There is no max value for Cfly so you can use other values but I would recommend to use the value suggested in datasheet.

    There is no figure showing Iout vs Cfly and the datasheet says this on page 3, so your recommendation is 3.3uF, but my customer can use 22uF for Vin=5V, Vout=-5V and Iout=27mA?

    TPS60402-Q1 3.3 Fixed frequency 50 kHz

    > More information for Rswitch can be evaluated through figure 9.

    Do you mean my customer checks the figure 9 to find the output resistance(Ro) and figure out the Rswitch from the calculation (2) on page 11?

    Best Regards,

    Yoshikazu Kawasaki

  • Hi Yoshikazu,

    The data characterized for output resistance is for 3.3uF Cfly. Higher Cfly value has negligible effect. Could you explain what is the purpose of this value?

    Best regards,
    Tanvee
  • Hello Tanvee,

    The background of the 22uF is to reduce noise as much as possible becuase the IC will be used in audio products. Do you think you can support Vin=5V, Vout=-5V and Iout=27mA at Cfly=3.3uF?

    Best Regards,
    Yoshukazu Kawasaki
  • Hi Yoshukazu,

    Yes, the device can support max. 60mA as suggested in datasheet. Higher Cfly value with lower impedance might improve the voltage drop but the output resistance is not measured for that value.

    Best regards,
    Tanvee