JAJS627M april 2000 – may 2023 LM2676
PRODUCTION DATA
The output capacitor acts to smooth the DC output voltage and also provides energy storage. Selection of an output capacitor, with an associated equivalent series resistance (ESR), impacts both the amount of output ripple voltage and stability of the control loop.
The output ripple voltage of the power supply is the product of the capacitor ESR and the inductor ripple current. The capacitor types recommended in the tables were selected for having low ESR ratings.
In addition, both surface mount tantalum capacitors and through-hole aluminum electrolytic capacitors are offered as solutions.
Impacting frequency stability of the overall control loop and the output capacitance, in conjunction with the inductor, creates a double pole inside the feedback loop. In addition, the capacitance and the ESR value create a zero. These frequency response effects together, with the internal frequency compensation circuitry of the LM2676, modify the gain and phase shift of the closed-loop system.
As a general rule for stable switching regulator circuits, it is desired to have the unity gain bandwidth of the circuit to be limited to no more than one-sixth of the controller switching frequency. With the fixed 260-kHz switching frequency of the LM2676, the output capacitor is selected to provide a unity gain bandwidth of 40 kHz (maximum). Each recommended capacitor value has been chosen to achieve this result.
In some cases, multiple capacitors are required either to reduce the ESR of the output capacitor, to minimize output ripple (a ripple voltage of 1% of VOUT or less is the assumed performance condition), or to increase the output capacitance to reduce the closed-loop unity gain bandwidth to less than 40 kHz. When parallel combinations of capacitors are required, it has been assumed that each capacitor is the exact same part type.
The RMS current and working voltage (WV) ratings of the output capacitor are also important considerations. In a typical step-down switching regulator, the inductor ripple current (set to be no more than 30% of the maximum load current by the inductor selection) is the current that flows through the output capacitor. The capacitor RMS current rating must be greater than this ripple current. The voltage rating of the output capacitor must be greater than 1.3 times the maximum output voltage of the power supply. If operation of the system at elevated temperatures is required, the capacitor voltage rating can be de-rated to less than the nominal room temperature rating. Careful inspection of the manufacturer's specification for de-rating of working voltage with temperature is important.