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[FAQ] TPS546D24A: Selecting Rboot and Calculating Rboot power dissipation

Peter James Miller
Guru 50795 points
Part Number: TPS546D24A

In the TPS546D24A EVM, and many TI reference designs for high-current Buck switching converters, I see that there is a resistor in series with the BOOT pin to connect a capacitor to the switching (SW, PHASE, Etc) pin.

How do I calculate the value of Rboot resistor?

How do I calculate the power dissipation in the Rboot resistor so I can correctly size the resistor?

  • 0
    TI__Guru 50795 points

    he BOOT pin of a synchronous BUCK converter like the TPS546D24A (and many others) powers the circuitry of a "floating" gate driver for the high-side FET and is connected to the SW pin with a capacitor to maintain the BOOT to SW voltage when the high-side FET is turned on.  This "Boot Strap" power supply maintains a loosely regulated voltage from BOOT to SW for driving N-channel MOSEFTs that need a gate drive voltage that exceeds the power input voltage PVIN.

    Adding a resistor in series with BOOT increases the total resistance in series with the Boot Capacitor (voltage source for gate drive) and the high-side GATE to reduce the gate drive current and slow the rise-time of the high-side FET to reduce ringing.  While most designs should not require a BOOT resistor, one can be added to most designs to reduce SW rising edge ringing to reduce SW to PGND voltage stress and high-frequency EMI generated by the switching node.

    Because the voltage drop on the boot resistor reduces the circuit voltage on the BOOT pin during the high-side gate turn-on, the series boot resistor should be kept as low as practical while meeting switch voltage stress and EMI requirements of the specific design.  In addition, the BOOT resistor should not be greater than the internal drive pull-up resistance.  For the TPS546x24A family, that is 4.2Ω for PVIN less than 14.5V and 10Ω for PVIN greater than 14.5V.

    The BOOT pin draws high peak currents that can be a few Amperes, but only during the short duration when the high-side FET is turning on.  The Boot resistor, Rboot, limits this current to control the slew-rate, ringing, and peak voltage stresses during the turn-on of the high-side FET, which can complicate estimating power dissipation.

    The approximate power dissipation is given by:

    Pdiss = R * I^2 * Sqrt ( D)

    Rboot * [ (Vdrive - Vplat) / (Rdrive + Vboot) ] ^2 * [ Qg * (Rdrive + Rboot) / { (Vdrive - Vplat) * Tsw } ] 

    Unfortunately, more integrated FET devices, like the TPS546D24A, do not include factors like Qg, Vdrive, Rdrive or Vplat, because the power dissipation Rboot is generally neglectable, even for an 0402 resistor.

    For the TPS546D24A, the approximate values are:

    Vdrive = 4.7V, Vplat = 2.0V, Qg = 6.5nC and Rdrive = 4.2-Ohms

    Solving this for a couple of different resistor values at 1.5MHz:

    1-Ohm:  1 * (2.7V / 5.2-Ohms)^2 * ( 6.5nC * 5.2-Ohm / (2.7V * 667ns)) = 5mW

    4.7-Ohm: 14mW

    10-Ohm (Not recommended) 18.5mW