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UCC27284: the spec of output current, output resistance and internal diode

Shinichi Inoue
Guru 22106 points

Part Number: UCC27284

Dear Specialists,

My customer is considering UCC27284 and have questions.

I would be grateful if you could advise.

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[1] P1 of the UCC27284 data sheet mentions a peak output current of ±3A, but the absolute maximum ratings section does not mention the output current of the HO and LO pins.
Does the UCC27284 have an output current limit as an absolute maximum rating?
I would like to know the absolute maximum rating value

[2] Or is the output current limited as a result of the thermal limit of Tj-40 to 150degree that exists in the absolute maximum ratings?

[3] Regarding the output resistance of HO and LO of UCC27284, the data sheet only describes the output resistance value (up to about 4Ω) obtained from the voltage when 100mA is applied.
Is there any change in the output resistance from a maximum of about 4Ω when the output current increases?

[4] Is there any limit to the current flowing through the bootstrap diode as long as the temperature of the UCC27284 is within the range of Tj-40 to 150°C?

The current that flows through the built-in bootstrap diode of the UCC27284 varies depending on the ON resistance of the half-bridge low-side FET, the OFF→ON time, the capacitance of the bootstrap capacitor, and other factors.
However, there is no absolute maximum rating for bootstrap diode current.

For example, if the ON resistance of the FET is 50 mΩ, the OFF→ON time is 10 ns, and the bootstrap capacitor is 1.0 μF, the peak current of the bootstrap diode will be considerably large when the power is turned OFF→ON.
"In that case, how should the power consumption of the bootstrap diode be estimated?"

[5] The equivalent resistance value of the built-in bootstrap capacitor, Rd, should be considered to be Max.2.5Ω even if the current value deviates greatly from 100mA

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I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Mastermind 23215 points

    Hello Schinichi,

    I have a summary of your questions below and will address each one.

    [1] P1 of the UCC27284 data sheet mentions a peak output current of ±3A, but the absolute maximum ratings section does not mention the output current of the HO and LO pins.
    Does the UCC27284 have an output current limit as an absolute maximum rating?
    I would like to know the absolute maximum rating value.

    Answer: The peak output current of the driver stage is inherently limited by the internal devices and pre drive circuit. The driver can support driving a capacitive load, or equivalent Mosfet gate charge with no external gate resistance and not over stress the driver output stage.

    [2] Or is the output current limited as a result of the thermal limit of Tj-40 to 150degree that exists in the absolute maximum ratings?

    Answer: The driver peak current is internally limited as mentioned in 1 above. However review the applications example in the datasheet to confirm the power dissipation and thermal rise of the application to confirm there is not excessive gate drive loss in the driver. This is more of an average power issue.

    [3] Regarding the output resistance of HO and LO of UCC27284, the data sheet only describes the output resistance value (up to about 4Ω) obtained from the voltage when 100mA is applied.
    Is there any change in the output resistance from a maximum of about 4Ω when the output current increases?

    Answer: The output resistance of the internal Mosfet will stay consistent over higher output drive current ranges. When the driver output peak current is achieved the voltage drop across the internal device will begin to be higher than just current x Rdson however.

    [4] Is there any limit to the current flowing through the bootstrap diode as long as the temperature of the UCC27284 is within the range of Tj-40 to 150°C?

    The current that flows through the built-in bootstrap diode of the UCC27284 varies depending on the ON resistance of the half-bridge low-side FET, the OFF→ON time, the capacitance of the bootstrap capacitor, and other factors.
    However, there is no absolute maximum rating for bootstrap diode current.

    For example, if the ON resistance of the FET is 50 mΩ, the OFF→ON time is 10 ns, and the bootstrap capacitor is 1.0 μF, the peak current of the bootstrap diode will be considerably large when the power is turned OFF→ON.
    "In that case, how should the power consumption of the bootstrap diode be estimated?"

    Answer: For the bootstrap diode, the peak current that exists when initially charging the bootstrap capacitor can be high but the IC has been confirmed to not have any stress issues with peak forward current. For a maximum current rating we recommend considering using an external boot diode when the average bootstrap diode current is beyond 100mA. This can be estimated by looking at the average HB current from driving the total Qg on the HO output. I_hbavg=fsw x Qg

    [5] The equivalent resistance value of the built-in bootstrap capacitor, Rd, should be considered to be Max.2.5Ω even if the current value deviates greatly from 100mA

    Answer: I assume you are referring to the diode dynamic resistance, can you confirm, and what you mean if the current deviates greatly, are you interested in higher currents?

    Regards,