External components value re-use

Question

Hi, Looking at the schematics at: wizwiki.net/wiki/doku.php?id=pro … fschematic there are many unique component values. My application uses an RJ45 with integrated transformer and separate CT signals for RX and TX. Let’s start with the two 6.8nF series capacitors. The headline “RJ45 with integrated Transformer and connected CT” states the reason for adding the series 6.8nF capacitors. They are needed to DC isolate the RX pins from the common TX/RX CT which will have a DC voltage of about 3.3V (through the 10 Ohm resistor). However, by looking at the schematic “W5500 Ref.Schematic - RJ45 with Transformer” the 6.8nF capacitors are still included. The RJ45 in this schematic has separate RX and TX CT signals - i.e. they are NOT ONE common. My question then is: Why are those 6.8nF capacitors included here? If there is another good reason for having them (explain why please), will 10nF do the job (i.e. can I replace 6.8nF to 10nF)? This would be beneficial since 10nF is used extensively elsewhere in the schematic. Another ambiguous capacitor value is the 22nF to GND from TCT. This same capacitor is replaced with a 100nF when CT are common. Will a 100nF capacitor still be usable for this capacitor when using separate CT? Again, this would be beneficial in order to reduce the unique component part values. There is also the TOCAP 4.7uF capacitor. Since 10uF us used extensively elsewhere. Will a 10uF work instead of the 4.7uF part? Best Regards, HC

hctiming · Answer

Is there any application engineer from wiznet who can answer these questions?

bangbh · Answer

Hello, HC. I’m peter employee of wiznet. Sorry for late. In your application board(seperate CT), you can change 6.8nF to 10nF or 0ohm resister. It works well. 22nF can changed to 10nF or 100nF. It works well. External capacitor 10nF and 4.7uF(with the accuraty of 5%) is fixed value. Please do not change the capacitance. Those capacitors is for internal LDO compensation. 1 and 2 work well but it does not tested in all situation. So we can’t guarantee about above mentioned. Best regards peter.

hctiming · Answer

Hi Peter, Thank you for the clarification. So, this means that your schematic I referred to has included capacitors that there is actually no need for - to my understanding. Regarding your 3’rd statement: You specify that the 4.7 uF capacitor must be within 5% tolerance. I have difficulties understanding that this needs to be such an accurate value. First, by searching for 5% ceramic capacitors from mouser: no.mouser.com/Passive-Components … frZ1z0z7ym The smallest physical size (inch) is 1206, which is quite big. Also, since high dielectric capacitors tend to change their nominal value by DC bias: murata.com/en-sg/support/faq … /char/0005 The 5% requirement seems, to my understanding, not to be reasonable. Please consult the designer of the internal LDO in order to justify the 5% requirement if possible. Best Regards, HC

bangbh · Answer

You’re right. 4.7uF 5% capacitor is quite big and expensive. 5% tolerance is recomended value not absolute.You can use 10% tolerance capacitor. Best regards. Peter.