Capacitance (C): The capacitance value determines the amount of charge the capacitor can store and deliver. It is typically specified in farads (F). The required capacitance depends on the load current, output voltage ripple, and desired voltage regulation.
Voltage Rating (V): The voltage rating of the capacitor should be higher than the maximum voltage across it in the buck converter circuit. It ensures that the capacitor can withstand the voltage stress without failing.
Equivalent Series Resistance (ESR): Capacitors have internal resistance, known as ESR, which affects their ability to filter out voltage ripples. Lower ESR is desirable for minimizing output voltage ripple and improving efficiency. It is specified in ohms (Ω).
Ripple Current Rating: The capacitor should be able to handle the ripple current flowing through it without significant performance degradation. Ripple current is caused by the fluctuating current in the buck converter circuit. Ensure that the capacitor ripple current rating is higher than the actual ripple current.
Temperature Coefficient: The temperature coefficient indicates how the capacitance value changes with temperature. It is specified in parts per million per degree Celsius (ppm/°C). Select a capacitor with a low temperature coefficient to maintain stable performance over a wide temperature range.
Package Size: Consider the physical dimensions and package size of the capacitor to ensure it fits within the space constraints of your design.
Cost: Cost is an important consideration for component selection. Capacitor prices vary based on parameters such as capacitance, voltage rating, and manufacturer. Choose a capacitor that meets your requirements while also fitting your budget.
Design parameters:
• Output voltage, VO = 1.2 V
• Maximum load current, IO = 6 A
• Estimated efficiency at maximum load, h = 87%
• Switching frequency, fSW = 600 kHz
• DC input bus voltage = 12 V with 5% tolerance
• Worst-case maximum input voltage, VIN_max = 16 V
• Bus converter control bandwidth = 6 kHz
• Transient load step, IStep = 3 A
• Worst-case board temperature = 75°C
Design requirements:
• Allowed input peak-to-peak ripple voltage,
ΔVIN_PP ≤ 0.24 V
• Allowed input transient undershoot or overshoot,
ΔVIN_Tran ≤ 0.36 V
The capacitor voltage rating should meet reliability and
safety requirements. For this example, all input capacitors
are rated at 25 V or above.
Calculating Ceramic Capacitance
Use Following Formula to determine the amount of ceramic capacitance required to reduce the ripple voltage amplitude to acceptable levels:
where ,
fSW is the switching frequency in kHz
IOUT is the steady state output load current
CMIN(1) is the minimum required ceramic input capacitance in μF
(Some of CMIN is supplied by the module)
VP(max) is the maximum allowed peak-peak ripple voltage dc is the duty cycle (as defined above)
Conclusion:
In any power system, input and output capacitance is key to optimum performance. Good engineering practice requires that additional external capacitance be placed at the input and output of all regulators.
A well designed power supply decoupling network will employ different types of capacitors. System design requirements will determine the amount and type of capacitors for any design.
Detailed analysis has been performed to allow capacitor limits to be accurately defined. By following the capacitor recommendations in the data sheet and selecting capacitors based on your actual operating conditions, a reliable, low-cost power system can be designed.
Reference Documents :
SLTA055–Input and Output Capacitor Selection
TI-Analog Applications Journal
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