1. Mixed-Type Output Capacitors & Noise Mitigation
Akshay Mehta
Texas Instruments

2. Output ripple components
Low Frequency (LF) “Ripple” & High Frequency (HF) “Noise”

3. LF ripple
Low Frequency (LF) “Ripple”

4. LF ripple: Origin
A function of the inductor ripple current & the output capacitor impedance (formed by C, ESR, and ESL)

5. Aluminum electrolytics - Overview
Low cost
Mature technology with low cost materials
Long history
Industry started in the 1930’s
Many manufacturers to choose from
High capacitance values available
Only choice for SMPS that need high voltage & high capacitance
Highest ESR
Low frequency capacitance roll off due to higher ESR

6. Aluminum electrolytics - Disadvantages
Large swings in ESR vs. temperature
Cold temps have 4-8x higher ESR than at room temperature

7. Frequency response of a 100µF electrolytic capacitor with 100mΩ ESR
High ESR at low frequencies
The high ESR introduces a zero at low frequencies which may cause instabilities during a transient if not properly compensated for
The high ESR will also increase the steady state ripple
To mitigate this & reduce LF ripple, parallel ceramic caps can be added.

8. Ceramics - Overview Voltage bias effect Lowest cost devices
Best choice for local bypassing
Not polarized
Low series resistance (Low ESR)
Low effective series inductance (Low ESL)
Significant effects for Class 2 dielectrics i.e. X5R, X7R
Voltage bias effect
Temperature effects
High Q
Frequency selective

9. Adding a parallel 2.2µF ceramic capacitor to the 100µF electrolytic
𝑍= 1+𝑠 𝑅 1 𝐶 𝑠 𝑅 2 𝐶 2 𝑠 𝐶 𝑡 1+𝑠 𝑅 𝑡 𝐶 𝑠
𝐶 𝑠 = 𝐶 1 ∗ 𝐶 2 𝐶 𝑡
𝐶 𝑡 = 𝐶 1 + 𝐶 2
𝑅 𝑡 = 𝑅 1 + 𝑅 2
Ceramic capacitor ESR is at a very high frequency
Addition of ceramic cap introduces a pole at low frequencies
This will reduce the effect of the ESR zero of the electrolytic cap
Effective ESR of the parallel combination reduces, thus reducing the steady state ripple

10. How much is enough?
Addition of a single 2.2µF ceramic cap introduced a pole, but was it at a low enough frequency?
How much was the ripple reduced by? Was it enough?
Is the design stable?
These are some questions that can be answered after doing some analysis on the parallel combination of the mixed type capacitors.

11. Reducing LF ripple with mixed type output caps for buck topologyXC
Impedance of a capacitor with ESR can be visualized as shown
It is defined as: Z
𝑍 = 𝑅 2 + 𝑋𝐶 2 and 𝜃 𝑍 =𝑎𝑡𝑎𝑛 𝑋𝐶 𝑅
𝑍= 𝑍 𝑒 𝑗𝜃 𝑍
where θ R
For two capacitors in parallel, the magnitude & phase can be written as follows:
𝑍𝑐 = 𝑅 𝑋 𝑐1 2 ∗ 𝑅 𝑋 𝑐 𝑅 1 + 𝑅 𝑋 𝑐1 + 𝑋 𝑐 and 𝜃 𝑍 𝐶 =𝑎𝑡𝑎𝑛 𝑋 𝑐1 𝑅 1 +𝑎𝑡𝑎𝑛 𝑋 𝑐2 𝑅 2 −𝑎𝑡𝑎𝑛 𝑋 𝑐1 + 𝑋 𝑐2 𝑅 1 + 𝑅 2
The effective ESR and capacitance can then be estimated as:
𝑹 𝒆𝒇𝒇 = 𝒁 𝑪 ∗ 𝒄𝒐𝒔 𝜽 𝒁 𝑪 𝑪 𝒆𝒇𝒇 = 𝟏 𝟐∗𝝅∗𝒇∗ 𝒁 𝑪 ∗ 𝒔𝒊𝒏 (𝜽 𝒁 𝑪 )
Ceff and Reff are dependent on frequency, f
Spreadsheet to estimate output ripple
𝑽 𝑶 𝒓𝒊𝒑𝒑𝒍𝒆 ͌ 𝑰 𝑳𝒑𝒑 ∗ 𝑹 𝒆𝒇𝒇 𝟐 + 𝑰 𝑳𝒑𝒑 𝟖∗𝒇∗ 𝑪 𝒆𝒇𝒇 𝟐
Output voltage ripple for buck:

12. LF output voltage ripple with 1A ripple current

13. Ceramic cap: Voltage bias effect including case size
X5R, 16V Rated Capacitors
1uF 0603
1uF 0805
1uF 1206
Capacitance decreases more quickly with smaller Case sizes

14. Ceramic caps: Choose the right rated component
If chosen cap has rated voltage close to applied voltage, the capacitance derates
This will push the pole out to higher frequencies & will not be as effective in terms of compensating for the ESR of the electrolytic cap
For two capacitors in parallel the transfer function is 𝑍= 1+𝑠 𝑅 1 𝐶 𝑠 𝑅 2 𝐶 2 𝑠 𝐶 𝑡 1+𝑠 𝑅 𝑡 𝐶 𝑠 .
There is an additional pole caused by addition of ceramic capacitance defined by 𝑓 𝑝𝑜𝑙𝑒 = 1 2𝜋 𝑅 𝑡 𝐶 𝑠 . Rt is the total ESR and Cs is the series combination two capacitances.

15. Ceramic caps: Y5V dielectric characteristics
DO NOT USE! Y5V & Z5U ceramic dielectrics for power supply designs

16. HF noise: Origin
Fast edge of switch node voltage couples right through the inductor parasitic capacitance
HF noise with Inductor 1
Ringing frequency in 100’s of MHz, modulated by the inductor & output cap parasitics
HF noise with Inductor 2 (more parasitic C)

17. Measuring output ripple

18. Measuring output ripple

19. Measuring output ripple

20. Measuring output ripple

21. Measuring output ripple
When taking VOUT measurements, don’t probe any other noisy nodes (e.g. SW) with the scope
VOUT and VSW measured simultaneously
Only VOUT measured

22. Measuring output ripple – Custom 1x probe
0.1µF AC coupling (DC blocking) capacitor & external (or internal) 50Ω termination at the scope
High Pass cutoff frequency at 1 / (2πRC) = 32kHz
This simple probe is suitable for use for up to 250MHz BW setting on the scope

23. Measuring output ripple – Custom 1x probe

24. Reducing output ripple (HF)
Buck input capacitor placement – reduce source of ringing

25. Reducing output ripple (HF)
Buck input capacitor placement – reduce source of ringing

26. Reducing output ripple (HF)
FREE!
Buck input capacitor placement – reduce source of ringing

27. Reducing output ripple (HF)
After careful input capacitor placement, we get what we get
There is still some HF noise on the output
Now we need to filter the left over HF spike
We need to provide “low” impedance path at the frequency of interest
Method with capacitors only:
Leave empty small capacitor footprint(s) in your layout
When the hardware arrives, measure the HF ringing frequency with high BW scope setting
Remember, the ringing frequency is a result of the inductor parasitics, PCB layout, output capacitor, so knowing the frequency before getting hardware is difficult
Once the frequency is known from measurement, pick a small ceramic capacitor with a “notch” or ”null” at the frequency of interest
Place 1-2 small capacitors in parallel to reduce the HF noise

28. Reducing output ripple (HF)
Ceramic capacitor impedance (plot from manufacturer software)

29. Reducing output ripple (HF)
Measure your caps
Mark up your capacitor kit!

30. Gen6 SIMPLE SWITCHER® easy-to-use pin out & schematic
Wide VIN range 3.5V to 36V or 60V
Low BOM count
Good light load efficiency with DCM / PFM
27 µA operating quiescent current
Full-featured: Soft Start, Tracking, Sync, Fs, PGOOD, UVLO, OCP, TSD

31. Pin-out optimized for EMI performance
GND
VIN Bypass
GND
Passes CISPR22 class B without filter
VOUT
VIN
Gate Drive FB
The pin configuration design of the synchronous SIMPLE SWITCHER family is optimized for easy PCB layout and best EMI performance.
PVIN is next to PGND pins, allowing Cin right next to pins, to contain switching noise in a as small as possible area, thus lower radiated EMI.
CBOOT is next to SW pins, boot cap right next to pins, reduce noise from gate drive path.
FB pin is place far away from power switches, and an AVIN pin right next to it, providing some shielding. The divider resistors can be placed right next to these two pins. FB path can be very small and immune from noises.

32. LM43603 EMI performance EMI curves on all 7 datasheets Radiated
Conducted
Measured on the LM43603PWPEVM with default BOM
No input filter used.
Measured on the LM43603PWPEVM with default BOM
Input filter: LIN = 1 μH
Cd = 47 μF CIN4 = 68 μF
We’ve tested all seven of our products for radiated and conducted EMI on the standard EVM in external standard lab.
All boards passed radiated EMI CISRP22 class B without additional filtering.
For conducted, we provide the filter values in every datasheet for the curves shown.

33. LM43603 – 36VIN 3A synchronous buck Ta=25°C, EVM, 500kHz
LM43603EVM
VIN=12V VOUT=3.3V IOUT=3A
LM43603EVM
VIN=24V VOUT=3.3V IOUT=3A IC IC IC
LM43603EVM
VIN=12V VOUT=3.3V IOUT=2A
LM43603EVM
VIN=24V VOUT=3.3V IOUT=2A IC IC
50mm x 50mm, 4 Layers

34. LM46002 – 60VIN 2A synchronous buck Ta=25°C, EVM, 500kHz
LM46002EVM
VIN=12V VOUT=3.3V IOUT=2A
LM46002EVM
VIN=24V VOUT=3.3V IOUT=2A
LM46002EVM
VIN=48V VOUT=3.3V IOUT=2A IC IC IC

35. Flexible features for broad-market applications
Enable / UVLO
PGOOD
SS / Tracking / Prebias
Programmable fS: 200kHz to 2.2MHz
Frequency synchronization
Internal Compensation
OCP / SC / TSD
HTSSOP package
EVM tested under EN55022/CISPR22 EMI standards
This family is full featured.
Precision enable, UVLO, Pgood flag, start up control, prebiased, frequency is pin programmable by a resistor from 200k to 2.2MHz, and SYNC to ext clock.
We allow the pins to be floating when the feature is not in use to minimize BOM count.
When pin floating, the default frequency is 500kHz, and soft start time is about 4ms.
Compensation is internal, Cff can be used to optimize transient performance with different types of output capacitors
Protection includes over current protection, short circuit protection (hiccup) and thermal shutdown
HTSSOP leaded package with exposed pad is used for ease of use and better thermal performance
Other features about the family, I will discuss in the coming slides…
More details for pin-2-pin compatible family, EMI performance, thermal performance and Webench tool are shown in the next slides.
AECQ qualification is also planned. 35 35

36. Fore more information: SIMPLESWITCHER.com
Thanks!
Fore more information:
SIMPLESWITCHER.com