LT3845 Internal Oscillator Frequency requirement for Sync operation

The LT3845 datasheet's SYNC pin function description states that the internal oscillator frequency is 10% to 25% below the external clock frequency. Later on in the datasheet in the Oscillator SYNC applications information it states the oscillator is at least 10% below the desired sync frequency.

Is the actual requirement that the internal oscillator frequency be at least 10% lower than the external clock frequency?

The reason I ask is if the requirement is 10% to 25% below the external clock frequency, then the requirement may not be met for 200kHz with just the tolerance for the operating frequency. The operating frequency for Rset = 49.9kohms is specified as 270kHz minimum, 300kHz typical, and 330kHz in the electrical characteristics table which is +/-10%. If the internal oscillator frequency is set to 165kHz which is 17.5% below the external clock frequency of 200kHz, then +10% is 181.5kHz and -10% is 148.5kHz. This range is outside the 150kHz to 180kHz 10% to 25% below the external clock frequency requirement. This is not counting any tolerances Rset has that would further extend the range. There is actually no solution of setting the internal clock frequency with the +/-10% operating frequency tolerance to meet 10% to 25% below 200kHz.

Parents
  • 0
    •  Analog Employees 
    on Apr 8, 2019 8:01 PM

    The sync signal functions as an oscillator ramp termination, meaning the sync pulse cuts short the internal oscillator voltage ramp. Therefore, the sync frequency has to be faster than the internal oscillator frequency.  The internal oscillator ramp is used to generate the slope compensation ramp, therefore, the external frequency does not want to deviate too far away from the internal oscillator. This is where the 25% guideline came from.  

Reply
  • 0
    •  Analog Employees 
    on Apr 8, 2019 8:01 PM

    The sync signal functions as an oscillator ramp termination, meaning the sync pulse cuts short the internal oscillator voltage ramp. Therefore, the sync frequency has to be faster than the internal oscillator frequency.  The internal oscillator ramp is used to generate the slope compensation ramp, therefore, the external frequency does not want to deviate too far away from the internal oscillator. This is where the 25% guideline came from.  

Children
No Data