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UC1845A-EP: Oscillator max frequency

Part Number: UC1845A-EP
Other Parts Discussed in Thread: UC1843A, UC1845A, UC1842A, UC1844A, UC1843A-EP

Does the UC1845A-EP support a 1MHz oscillator frequency as shown in Figure 6 of the datasheet?

The title page refers to "500kHz" operation but it is not clear if the divide-by-2 is considered when stating 500kHz.

  • Hi Steve,

    In the recommended table of the datasheet, we can see that the recommended maximum oscillator frequency is 500 kHz. Output frequency equals oscillator frequency for the UC1842A and UC1843A. Output frequency is one half oscillator frequency for the UC1844A and UC1845A. 

    TI recommends using capacitance values from 470 pF to 4.7 nF. TI also recommends that the timing resistor values chosen be from 5 kΩ to 100 kΩ. With the recommended RT/CT range, the frequency of oscillator doesn't go up to 1 MHz in the figure 6.



  • Teng -

    Thanks but please clarify where the RC values & 500kHz oscillator limit is specified - I do not see that in the UC1845A-EP datasheet (note this is a different datasheet than the regular UC1845A device). The datasheet I see from is SGLS134D, Revised January 2013.

    Also figure 6 of the UC1845A-EP datasheet shows operation @ ~ 800kHz fosc with 5kohms and 470pF.

    Please verify if the UC1843A-EP can operate at up to 800kHz oscillator frequency.


  • Hi Steve,

    Sorry for that I found them in UC1845A datasheet, but I think UC1845A and UC1845A-EP should have same recommending operating switching frequency.

    Yes from the figure 6 of the datasheet, we can see that the oscillator frequency is ~730 kHz, but we don't recommend use the minimum RT and minimum CT together.

    The recommended maximum operating switching frequency is applied for all UC184XA-EP.

    But for UC1843A-EP, its output frequency equals to its oscillator frequency, so we can run it at the maximum PWM frequency of 500kHz, while UC1845A-EP can only run at the maximum PWM frequency of 250 kHz.