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TPS61379-Q1: Voltage Swing on the Vcc Pin?

James Olson
Prodigy 210 points
Part Number: TPS61379-Q1


Tool/software:

I'm trying to choose the capacitor that bypasses the Vcc pin of TPS61379. 

I chose 220nF for the bootstrap capacitor.  The application prose in the datasheet mentions that the capacitance of the Vcc capacitor needs to be at least 10x the capacitance of the bootstrap cap.  In my case this would point toward the Vcc cap being 2.2uF.

I'm trying to determine whether I can make the Vcc cap a 0402 device.  But it occurs to me that the voltage on the Vcc pin might be far higher than the input voltage for the IC.  I believe that the boot strap cap pumps Vcc up above VIN, correct?

For a VIN roughly equal to the voltage on a fully charged lithium ion battery (~3.8V), what is the expected maximum voltage on Vcc?  My output voltage is expected to be 5.2VDC.

Could I have chosen the boot strap cap to be 100nF instead of 220nF?  This would make it easier to find a Vcc cap in a 0402 package.

Thanks from Indianapolis US.

  • 0
    TI__Guru 61581 points

    Hi James,

    The engineer is out of office. Please expect a reply till Tuesday.

    Best Regards,

    Stefan

  • 0
    TI__Guru 61581 points

    Hi James,

    The engineer is out of office. Please expect a reply till Tuesday.

    Best Regards,

    Stefan

  • 0 in reply to Stefan Schauer
    TI__Genius 16955 points

    Hi James,

    Thanks for reaching out.

    James Olson said:
    I believe that the boot strap cap pumps Vcc up above VIN, correct?

    No, boot strap cap is the driver voltage for upper MOS and it get voltage from VCC.

    That is why it need to be 10 times smaller than VCC.

    the Vcc voltage regulates from the higher one of the Vin and Vout and its target is 4.8V.

    In your case, the expected voltage at VCC is 4.8V as the Vout is 5.2V.

    It is acceptable to choose 100nF at boot and 1uF at VCC.

    Best Regards,

    Fergus

  • 0 in reply to Fergus He
    Prodigy 210 points

    Thanks for the reply, Fergus.  If I can parrot back to you an interpretation of what you wrote:

    1.  The VCC output is not (as I thought) a super-boosted output voltage that is used to fully enhance the channels of strong pull-up MOS transistors.  That bootstrapping function is mostly not associated with the VCC pin but rather the VCC pin provides a regulated voltage "pedestal" on which the bootstrap cap rests during switching cycles.

    Because the VCC rail serves as a charging pedestal for bootstrapping, the capacitance of the bootstrap and VCC caps are interrelated.  The VCC cap's value needs to be at least 10x the value of the bootstrap cap lest the bootstrapping process cause downward divots in the VCC rail during switching.

    Furthermore, 100nF and 1uF are acceptable values for the bootstrap and VCC caps, respectively.

    2.  The VCC pin gives access to an internal +4.8VDC power supply rail used in various locations of the chip.  Except to provide a pull up voltage for the IC's Power Good function, TI did mostly not intend for the VCC pin to be loaded by the external application circuit. 

    However, I notice in the datasheet an Icc=6mA output current spec that applies to Vout=9V operation, so maybe I'm wrong about this "no external use" and some loading of the VCC pin by external circuitry is permitted.

    3.  The internal linear regulator that drives the VCC output pin is powered by the greater of these two voltages: (1) VIN and (2) OUT.

    Thanks for the response.

    Jim Olson
    Curative Sound LLC
    Indianapolis, IN US


  • 0 in reply to James Olson
    TI__Genius 16955 points

    Hi Jim,

    Your descriptions are almost right.

    1. the VCC voltage charges bootstrap cap during the low side switch on time through a diode, and the bootstrap cap provide the gate driver for the upper switch when low side switch is off.

    2. the 6mA is the max output power of the inner LDO, it is usually recommended to limited external loading current within 3mA.

    3. totally right.

    Best Regards,

    Fergus