Other Parts Discussed in Thread: UC3854
Hi team,
My customers has questions on the UCC2818A-Q1 regarding the datasheet (see below).
Could you please help us to address them?
Thanks for your support.
Pol
----------------------------------------------------
1) §1 – Features
The input specifications of our application are the following :
- Switching frequency 200kHz
- Input voltage : 97-134Vrsm steady, 67 – 250Vrms 250ms transient
- Output power : 10W min , 20W max
- Output voltage : 12V
In our application we want to use a single stage isolated flyback converter for optimizing room.
In the features of the datasheet the component is able to control boost preregulators. Is also able to control single stage isolated flyback converter? Are maybe there some limitations ?
2) §6.4 - Electrical characteristics, Voltage Amplifier.
What is the difference between “input voltage” in the subsection ”VOLTAGE AMPLIFIER” and input voltage in the subsection ”VOLTAGE REFERENCE” ? Is it the same thing?
If the voltage amplifier is saturated, is it protected against phase reversal?
3) §6.4 - Electrical characteristics, Voltage Reference, Input voltage.
What is the min value? 7.313V or 7.687V ?
4) §6.4 - Electrical characteristics, Peak current limit.
In the block diagram, the “+” end of PKLMT comparator is referenced to a different ground symbol. Why?
5) §6.4 Oscillator.
With a typical switching frequency of 200kHz and a variation of 20% over temperature the range should be [160KHz;240kHz].
Is it the case? Or maybe the switching frequency is limited up to 220KHz (as mentioned in the features).
6) §6.4 - Electrical characteristics, Feed Forward.
Is [1.4V;5V] the recommended range for VFF ? Have you the absolute maximum ratings for VFF ?
7) §7.1 – Functional Block Diagram
- In application note U-134 the regulator UC3854 is used as reference. I noted that the more relevant difference between UC3854 and UCC2818-Q1 seems to be the current mirror 2:1. Is this modification an improvement? What kind of improvement?
- In the application note U-134 on page 3-285 a second order filter (two RC networks) can be implement at VFF PIN of UC3854. This should give a much faster response to changes in the RMS line voltage. The two poles of the filter can be placed at the same frequency.
Can I implement a similar 2 poles filter for the UCC2818-Q1? How?
8) §8.2.1.2. Soft start
Is TDELAY the time for bringing VVAOUT up to 7.5V? Or maybe is the supply of voltage amplifier increased up to 7.5V (according to the block diagram)?
9) §8.2.2 – Applications curves
- Figure 11 . K parameter seems to vary a lot for VAOUT<2V (low power).
Should I tune VAOUT=2V for the minimum operating power in my application ? Should VAOUT be higher than 2V for a correct operation?
- Figure 10 & 11. What is the RVFF value used to derive these curves?
- Have you a curve IMOUT(IAC) or k(IAC)?
