How to maximize PSRR (Power Supply Rejection Ratio)?

Better understanding PSRR

It happened across some forums in which people refer to configuring an ALS-HPULN in series with a DLS-HPULN with 1V (or more, depending on current need) voltage differential between both modules (or using the SLS-HPULN as an all-in-one power supply with two regulation stages in series). A key consideration is that for Analog's LT3045, Power Supply Ripple Rejection increases with voltage differential. It's also important to mention, that heat also increases with increasing voltage difference.

On page 10 of the LT3045 data sheet, "Power Supply Ripple Rejection" is around 60dB@.5V, 70dB@1V, 78dB@2V, 83dB@3V voltage differential (averaged over the charted frequencies). So there would be a 13dB ripple rejection improvement between stages if the voltage differential were changed from 1V to 3V.

Keep in mind that even 1dB ripple rejection is a significant amount because it's 26% ripple reduction, and -3dB is half the ripple.

For the following examples, lets assume we're starting with a fairly clean 10mV ripple. Going by the LT3045 data sheet, a 1V voltage differential generates 70dB ripple rejection with following result:

10mV ripple @ -70dB = 0.00316mV ripple

If we again pass that through another LT3045 at 1V voltage differential and another 70dB ripple rejection, we calculate following:

0.00316mV ripple @ -70dB = 0.000001mV ripple

Likewise, -70dB + -70dB = -140dB which is also 0.000001mV ripple from 10mV

Now lets start with a single LT3045 at 3V voltage differential and 83dB ripple rejection:

10mV ripple @ -83dB = 0.00070795mV ripple

So a single LT3045 at 3V voltage differential (-83dB ripple) is 4.5 times more effective at ripple rejection than a single LT3045 at 1V voltage differential (-70dB ripple).

But two series LT3045 at 1V voltage differentials is 708 times more effective at ripple rejection than a single LT3045 at 3V voltage differential, and it's 50% more efficient (net -2V versus -3V).

Also two series LT3045 at 1V voltage differentials is 3160 times more effective at ripple rejection than a single LT3045 at 1V voltage differential, but it's 50% less efficient (net -2V versus -1V).

So definitely series LT3045 has a distinct advantage over single.

So using power supplies in series can bring benefits, whereas it's absolutely necessary to respect the wasted power (which is directly transformed into heat) and take care about additional cooling in some cases, especially using HPULN modules with two rails / regulation stages, such as the MD-, the ALD, or the SLS-HPULN module. While the both first mentioned modules have two separate rails (that can be used in a series connection), the SLS-HPULN is a native "two rails in series" power supply offering one output with above described double regulation.

Our other blogs will be helpful to find application matching individual transformer voltage.