UC3843: RT/CT pin external component selection

Hi,

Rt and Ct form a time constant. if the constant too small, it requires much faster response from other circuit inside which may not be ok. Too big, some circuit cannot wait. So there is a range as constraint for a design of an IC in terms of time.

In reality, Figure 10 of the datasheet provides validated and recommended values for the constant. Outside of the figure is not recommended as the operation may not be as expected or simply cannot work.

Hi Given,

For the UC3843 RT/CT pin, the CT is charged by (VREF-VRTCT)/RT, so the charging current must be supplied by VREF source current capability. Limiting the minimum RT to 5 kΩ, limits the charging current to ~1mA which is a safe value for the VREF regulator. CT is discharged by an active pull down inside the RT/CT pin, "Osc" block shown in the block diagram, Figure 12 in the data sheet. The discharge current, i, sinking into the RT/CT pin is given by: i=CT*dV/dt. If CT is too large the current into the RT/CT pin is too high for the internal active pull down and this is why we limit the max CT to 100 nF. Make sure the customer follows the RT, CT limitations shown in the UC3843 data sheet. Choosing RT or CT values beyond the recommended limits can result in misbehavior of the oscillator and/or OUT gate drive signal or damage to the internal oscillator. If interested in more details about the UC3843 oscillator behavior, check out SLUA173. Thanks for connecting through E2E.

Regards,

Steve M