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# TIDA-01505: RTCT Pin

Part Number: TIDA-01505
Other Parts Discussed in Thread: UCC28C43

Is the Q15 transistor is necessary to connect in the circuit? If not what negative effect of that parameter Q15?

• Hi Suma,

Yes, it is needed. Q15 is to as buffer adding slope compensation. C67 can be removed

• Hi Xun,

1) Can I get the More details on the Slope compensation for primary side regulation and in the refence document how the below equation is derived?

2) Formula for calculating the timing resistor and Capacitor used in the Circuit R26 and C31?

• Hi Suma,

For 1) Slope compensation is required when duty cycle is exceeding 50% in order to suppress the sub-harmonic oscillation.

The component values are adjusted experimentally. Q15 is a voltage follower, R63 provides the load to Q15, 1k is chosen as rule of thumb to bias the emitter current. C67 is optional (purpose is for DC decoupling). R71 and R72 forms the resistive divider from RT/CT pin to the CS pin, the larger the value of R72 is, the bigger the compensation is added. The value of R71 is tuned under the condition of converter running at 50V input until the sub harmonic oscillation is away.

For 2) C26 and C31 set the oscillator frequency. You can check calculation in the UCC28C43 datasheet for the calculation.

www.ti.com/.../ucc28c43.pdf

• Hi Xun,

As mentioned in data sheet the C31 should be in the range of 220 pF to 4.7 nF. But the in reference C31 is 1000pF. Can you suggest how they have considered this value.

I am looking for answer in previously mentioned  equation is derived. If any reference document related to this question.

• Hi Suma,

There is no special consideration when choosing 1000pF (1nF), any value in between is fine. 1nF is a standard value to choose.

The equation on slope compensation is calculated according to equation (33) page 30 in the datasheet

www.ti.com/.../ucc28c43.pdf