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Solution Guide
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Solution Guide
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In the automotive field, CPUs and FPGAs for systems that require advanced image processing, such as onboard ADAS ECUs and autonomous ECUs, need to operate at high speed and require high drive current in conjunction with the increasing performance and functionality of systems. Also, in the ICT field, power supply configurations that support higher current required for devices that need huge amounts of power such as servers. There is a trend towards higher operating speeds and higher currents in the power supply lines of systems with increased performance and functions as described above. At the same time, power supply structures that maintain the nominal voltage within narrow tolerance ranges, which have decreased in conjunction with processor miniaturization, are also required.

Capacitor requirements within electrical characteristics

The equation and an image of voltage fluctuation at fixed loads and variable loads are indicated below.
In conjunction with shift to higher current (higher Δiout) and faster operation (higher dΔiout/dt), the range of voltage fluctuation when the load varies has increased in comparison with voltage fluctuation when under fixed load, and in order to control the fluctuation within the desired voltage range, a capacitor structure with high capacitance, low ESR, and low ESL is necessary.

Formula for calculating voltage fluctuation
Voltage fluctuation when under fixed load (ripple voltage) Voltage fluctuation when under variable load
ΔV out = ΔIL × ESR + ESL × Vin + ΔIL ΔV out = Δiout × ESR + ESL × dΔiout + 1 ∫ Δioutdt
L 8 × c × Fsw dt c
Image of voltage fluctuation during sudden change in load current

In cases where the current supplied by a DC-DC converter cannot follow the load fluctuation, the capacitor act as a backup, supplying the load with the current it needs until the power supply can catch up.

Many large capacitors, such as conductive polymer capacitors, have long been used in high-speed and high-current lines to stabilize the power supply and provide instantaneous power.
This document verifies the effectiveness of replacing conventional conductive polymer capacitors with MLCCs, which are characterized by low ESR and low ESL, to suppress voltage fluctuation including maintaining the stability (frequency characteristics) of the power supply.

Verification of optimal structures for output capacitors

Optimal output capacitor structures are verified for the two structures indicated in (1) and (2) below under the evaluation conditions specified below.
The following evaluation conditions and items are verified.

Output capacitor structure
Output capacitor structure
Total capacitance [μF] 990 1000
Conductive polymer capacitor
(2.5V 7343 330μF)
3pcs
MLCC
CGA6P1X7T0G107M250AC
(4.0V 3225 100μF)
Automotive products—Currently in mass production
10pcs

Evaluation conditions

  • ◆ Input voltage: 12V
  • ◆ Output voltage: 1.5V
  • ◆ Switching frequency: 400kHz
  • ◆ Load current (Δiout): 30A
  • ◆ Slew rate (Δiout/dt): 100A/μsec
Evaluation Items: Impedance/ESR characteristics

MLCCs have excellent ESR and ESL characteristics compared to conductive polymer capacitors.
ESR and ESL can be reduced by replacing conductive polymer capacitors with MLCCs.

Voltage fluctuation waveform when under fixed load and variable load

Voltage fluctuation when under fixed load and variable load can be controlled using the MLCCs in structure (2).
As indicated in the impedance/ESR characteristics, the MLCCs indicated in structure (2) make possible low ESR and low ESL and control voltage fluctuations.

Structure Conductive polymer capacitor MLCC(After adjustment)
Product / Specifications 2.5V 7343 330μF x3pcs CGA6P1X7T0G107M250AC x10pcs
4.0V 3225 100μF
Total capacitance [μF] 990 1000
Voltage fluctuation [mV]
When under fixed load

Δiout:30A
Voltage fluctuation when under load fluctuation (rising) [mV]

Δiout:0A→30A
Δiout/dt:100A/μs

Verification of optimal structures for output capacitors

It is shown that increasing the number of MLCCs is effective for suppressing voltage fluctuation, but in general, stability tends to decrease depending on the specifications of the power supply IC due to the effect of low ESR caused by the increase in the number of MLCCs. Because of this, it is important to confirm the relationship between responsiveness and stability of the power supply by acquiring and confirming a board diagram indicating the frequency characteristics of the power supply IC using a frequency response analyzer (FRA) or other such equipment. Also, stability can generally be adjusted by adjusting the constants of the capacitors and resistors in the external phase compensation circuit and feedback section of the power supply circuit block diagram as shown below.

The specific adjustment methods and so on will vary depending on the power supply IC used. Please contact the IC manufacturer directly regarding adjustment methods.

Example of measurement using a board diagram
Key points during measurement and effect on voltage fluctuation
Item Key Points Effect on voltage fluctuation
Crossover frequency Higher means faster operation
⇒ Effect on responsiveness
Reduction in voltage fluctuation
Phase margin/gain margin Higher means stable operation
⇒ Effect on stability
Prevents ringing and abnormal operation
Power supply circuit block diagram

Example of countermeasures in cases where margins are insufficient: Adjustment of phase compensation units

The waveform during variable voltage due to adjustment of the phase compensation unit is indicated below.
Compared to before adjustment, after adjustment, the voltage fluctuation is reduced by 31 mV by speeding up the crossover frequency from 43 kHz to 63 kHz.
Also, it can be seen that after adjustment, the phase margin increases from 30 deg to 53 deg and stability is improved due to the absence of the ringing wave form observed before adjustment. If a frequency response analyzer (FRA) is not available, stability can be determined by whether ringing or oscillation occurs during waveform observation, so check the stability during measurement.

MLCC phase compensation units Before adjustment After adjustment
Voltage fluctuation [mV]
When under fixed load

Δiout:30A
Voltage fluctuation when under load fluctuation (rising) [mV]

Δiout:0A→30A
Δiout/dt:100A/us
Whether ringing occurs is an indicator of stability

Verification of optimal structures for output capacitors: Summary

A summary of the evaluation results is set forth below.

  • Under a high-current, high-slew rate environment, voltage fluctuations during sudden load changes are greatly affected by the ESR and ESL components of the output capacitors.
    MLCCs make possible low ESR and low ESL and can control voltage fluctuations.
  • Generally, stability (the degree of phase margin) tends to decrease as a result of the low ESR of MLCCs, and in such cases, consider adjusting the constants of the phase compensation circuit.
  • When designing the structures of output capacitors, it is necessary to optimize the structure taking into consideration not just voltage fluctuation, but also the power supply stability.
  • TDK's high-capacity automotive MLCCs can be used in sets that require high reliability, ensuring better electrical characteristics and reliability.
  • TDK has an extensive lineup of components that can be selected to best meet the electrical performance requirements of the circuit used, mounting area restrictions, restrictions on the number of components, and other constraints. The TDK product lineup, data sheets, technical support tools, and more are available on TDK's websites.
Output capacitor structure Conductive polymer capacitor MLCC
Product / Specifications 2.5V 7343 330μF x3pcs CGA6P1X7T0G107M250AC x10pcs
4.0V 3225 100μF
Total capacitance [μF] 990 1000
Voltage fluctuation when under fixed load [mV]
Δiout:30A
61 12
(-80%)
Voltage fluctuation when under rising load [mV]
Δiout:30A
Δiout/dt:100A/μs
179 95
(-46%)