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UC2845: maximum output frequency

Howard Zou
Genius 12365 points
Part Number: UC2845
Other Parts Discussed in Thread: UCC28C45,

Hi,

for UC2845, the output frequency is 1/2 of oscillator frequency, and the oscillator frequency maximum is 500kHz, so the output frequency maximum is 250kHz, right?

But my customer find a board the output frequency of UC2845 is 400kHz, which means the oscillator frequency is 800kHz, which is larger than the datasheet's spec.

Is this okay?

  • 0
    TI__Mastermind 21765 points

    Hi Howard,

    The UC2845 max recommended  oscillator frequency is 500 kHz and this particular IC blanks every other output pulse, so the maximum OUT frequency is FOSC/2=250 KHz. This is the limit as specified from the data sheet. If your customer has programmed the oscillator for 800 kHz, they are 60% above the maximum. This is not okay, especially when we consider all the datasheet limits collectively! 800kHz is off the scale according to Fig 10 below.

    If you want to extend the frequency beyond 500kHz, you need to use the UCC28C45 which an enhanced BiCMOS p2p compatible with the legacy UC2845 version. UCC28C45 improvements are shown below as:

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    TI__Genius 12365 points

    Steve,

    thanks. We confirmed that the device used is UC2845AQ, whose maximum oscillator frequency is 500kHz, and output maximum frequency is 250kHz.

    But we also find that on the board, the actual output frequency reaches 430kHz and it works well.

    So in this case it's used beyond the datasheet's spec and we don't have any data to show how the device will perform at oscillator frequency=860kHz, right?