LMX2595: Schematic Review

Vamsi,

In general, this looks like a good strategy.  See comments below:

Regards,
Dean

Let's go through by Pin groups:

Vcc Pins:

Look Good

OSCin Pins:

Look good.  You have enough components to accommodate any scenario.  With the inductor, do be aware that if you reduce the slew rate of the OSCin signal too much, it can degrade the phase noise and spurs.

RAMPDIR,RAMPCLK, SYSREFREQ, and SYNC Pins

For these pins, you can float them or ground them if you don't need them;  they can all be powered down in software.  

There are other pins where routing is so critical, especially that the pull-up for the RFout pins needs to be as close as possible for the chip.  So if you can improve the layout elsewhere by not routing these, consider doing that.

RFoutB

This is close to RFoutA, where routing is absolutely critical, so you might consider floating these pins and powering down the chip if not used.

Some people feel more comfortable pulling this high to Vcc, but I have not found this to be necessary.  But even if you do this, you don't need all the other fancy decoupling as this will be powered down anyways.

RFoutAM

For the complementary side, the chip likes to see a similar load as it sees to RFoutAP.  It doesn't have to be perfect, but if it is terribly mismatched, this will lead to very high harmonics.  So what you do seems very reasonable.  Note that since the routing of RFoutAP is critical and the placement of pull-up R26 is critical to be close to the chip, what you can do is compromise the layout of RFoutAM to improve that for RFoutAP.  For instance, I was able to improve single-ended output power by making RFoutAP trace straight on the top layer, but took RFoutAM through a Via and put compoents on the bottom.  Our EVM for the LMX2595 sacrifices single-ended output power by giving equal treatment to RFoutAP and RFoutAM.

Hello Vamsi,

RAMPDIR and RAMPCLK are used only when ramping is needed. Please see "7.3.14 Ramping Function" section of the datasheet.

SYSREFREQ is used to generate a SYSREF signal. The use of this pin varies based on the SYSREF mode (repeater/pulsed/continuous). SYSREF is described in "7.3.15 SYSREF" section of the datasheet. Section "7.3.15.3 Examples" illustrates the differences between the modes.

Finally SYNC is used to create deterministic delay between the input and output of the device. Please see "7.3.11 Phase Synchronization". Sometimes PLLs are inherently synced and the SYNC pin does not need to be used. "Figure 33. Determining the SYNC Category" is a quick way to understand whether or not you need a  SYNC.

Thanks,

Vibhu

Hi Vamsi,

You can simply leave RFoutBP and RFoutBM pins floating and make OUTB_PD = 1.