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UC1843: Driving UC1843 RT/CT line directly from TTL external clock

Part Number: UC1843
Other Parts Discussed in Thread: UC3842,

  • SLAU143 , in Figure 23 shows a 555 timer driving the UC3842 (similar to the UC1843 that I am interested in ) driving the RT/CT pin directly, without a coupling cap.  The figure just above that on the same page shows a coupling cap between another example of an external clock source and the RT/CT pin.

    Do I need the coupling cap?  I am planning to drive that pin from a TTL clock (from a GAL 22V10 PLD) at 500 kHz.From TI App Note slua143- shows how to drive Sync.pdf

  • Hi Kenneth,

    Two methods are both OK. 

    If you use a small coupling capacitor and keep CT like above figure, the PWM can still run at the frequency set by RT and CT until the sync pulse appears, please note that the UC1843 oscillator must be set to a lower frequency than the sync pulse stream, typically 20 percent.

    But if you use below figure without a coupling capacitor, the PWM can't output pulse when there is no sync signal. The PWM can only work with the sync pulse.



  • Hi Teng,

    Thank you.

    The sync pulse, which is the external clock that I generate, is always there.  So if I drive the RT/CT pin without the coupling cap, the UC1843 will always be able to output PWM since the external clock is always present on the RT/CT pin.  Do you agree?

    Also what is the input resistance of the RT/CT pin? 


  • Hi Ken,

    Yes, you are right.

    I think you were just wondering the sink current at RT/CT pin when the sync signal is high. The discharge current of RT/CT pin is shown in figure 1 in the datasheet. Can it help you?