fSW limited by the minimum off-time, the variation in resistance across temperature must be considered when

using Equation 9. The selected fSW of 1.4 MHz is below the two calculated maximum values.

TBD

(9)

8.2.2.3 Choose the Inductor

To calculate the value of the output inductor (LOUT), use Equation 10. The output capacitor filters the inductor-

ripple current (IIND(ripple)). Therefore, selecting a high inductor-ripple current impacts the selection of the output

capacitor because the output capacitor must have a ripple-current rating equal to or greater than the inductor-

ripple current. On the other hand, larger ripple current increases output ripple voltage, but improves signal-to-

noise ratio and helps to stabilize operation. Generally speaking, the inductance value must set the ripple current

at approximately 15% to 40% of the maximum output current for a balanced performance.

TBD

(10)

TBD

(11)

TBD

(12)

TBD

(13)

8.2.2.4 Set the Current Limit (ILIM)

The RILIM resistor sets the valley current limit. Equation 14 calculates the recommended current limit target. This

includes the tolerance of the inductor and a factor of 0.85 for the tolerance of the current limit threshold. Equation

15 calculates the RILIM resistor to set the current limit. The typical valley current limit target is TBD A and the

closest standard value for RILIM is TBD kΩ.

TBD

(14)

TBD

(15)

With the current limit set, Equation 16 calculates the typical maximum output current at current limit. Equation 17

calculates the typical peak current at current limit. As mentioned in Choose the Inductor, the saturation behavior

of the inductor at the peak current during current limit must be considered. For worst case calculations, the

tolerance of the inductance and the current limit must be included.

TBD

(16)

TBD

(17)

8.2.2.5 Choose the Output Capacitor

There are three considerations for selecting the value of the output capacitor.

1. Stability

2. Steady state output voltage ripple

3. Regulator transient response to a change load current

First, calculate the minimum output capacitance based on these three requirements. Equation 18 calculates the

minimum capacitance to keep the LC double pole below 1/30th the fSW to meet stability requirements. This

requirement helps to keep the LC double pole close to the internal zero. Equation 19 calculates the minimum

capacitance to meet the steady state output voltage ripple requirement of TBD. This calculation is for CCM

operation and does not include the portion of the output voltage ripple caused by the ESR or ESL of the output

capacitors.

TBD

(18)

TBD

(19)

TPS54KB20

SLVSGP3 – MAY 2023

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