I designing a power supply +5V and -/+15V with ADP1613 and ADP1614 (+/-15V) for ADAS3022 and DAC LTC2668 from Linear Technology. I use ADP1613 to generate 6.5V from 5V input, then I use LDO to get clean 5V for analog circuit. After doing measurements I've noticed that on the output capacitor of ADP1613 I have 400mVpp noise. I was thinking about layout but I use ADP1614 part for +/-15V an on positive rail output capacitor (+15V) i have noise 10 times lower then on 6.5V rail and layout is almost the same as for 6.5V raill (ADP1613).
I've attached scope measurements from 6.5V and 15V rail for comparsion.
Design for 6.5V rail was generated with Analog tools ADP161x_SEPIC_Cuk_Designer (attached). The negative rail was not used. I was using ADP161x_SEPIC_Cuk because it allows to design output filter for positive rail. When I compare results from ADP161x_SEPIC_Cuk_designer and ADP161x_SEPIC_designer I see that it generates different values for Rc1, Cc1 and Cc2. Wrong Rc1, Cc1 or Cc2 can cause such noise on output ? Does my approach to design (using ADP161x_SEPIC_Cuk_desiner) is right ?
For my design 5V-CUK_SEPIC_designer.pdf was used.
Any suggestion are welcome.
The issue is the second stage output filter. Topologies with discontinuous output current (boost, SEPIC, flyback etc..) will have high noise spikes on the output because the current in the diode switches on and off very quickly and the output capacitor has some parasitic inductance. The solution is a second stage output filter as shown in the SEPIC-cuk design (L3 with Cout2 and damped by Rfilt). It is slightly confusing because the Cuk converter half of that converter is a continuous output current type of converter like and buck and therefore does not need the second filter. The trick with LC filters is to damp the so that it does not have high Q and oscillate. I recommend the following for the SEPIC. You currently have 2x 0805 10 uF caps on Cout1. Solder a 1 ohm resistor in series with one of them, add the same 1 uH inductor used in the SEPIC-Cuk design for L3, and then add another 2x 0805 10uF capacitors for Cout2. The resistor and capacitor in series provides the damping for the filter and the inductor L3 should block the high frequency noise.
yes, you are right. Problem was with output capacitor, by mistake I've soldered 2.2nF instead of 2.2uF. After changing Cout1 to 2.2uF (I willl test other values also) a get about 25mV noise on Cout1. After filter I have 2-3mVpp and after LDO I have noise under 1mV.
Thank you for your suggestion.
I've attached scope measurement for comparsion.