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f − Switching Frequency − kHz

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Selecting an Inductor Value

L +

(H)

(2)

Selecting the Output Capacitance

TPS40100

SLUS601–MAY 2005

APPLICATION INFORMATION (continued)

TIMING RESISTOR

TIMING RESISTOR

vs

vs

SWITCHING FREQUENCY

SWITCHING FREQUENCY

(250 kHz to 600 kHz)

(100 kHz to 350 kHz)

Figure 1.

Figure 2.

The inductor value determines the ripple current in the output capacitors and has an effect on the achievable

transient response. A large inductance decreases ripple current and output voltage ripple, but is physically larger

than a smaller inductance at the same current rating and limits output current slew rate more that a smaller

inductance would. A lower inductance increases ripple current and output voltage ripple, but is physically smaller

than a larger inductance at the same current rating. For most applications, a good compromise is selecting an

inductance value that gives a ripple current between 20% and 30% of the full load current of the converter. The

required inductance for a given ripple current can be found from:

where

•

L is the inductance value (H)

•

•

•

•∆I is the peak-to-peak ripple current in the inductor (A)

The required value for the output capacitance depends on the output ripple voltage requirements and the ripple

current in the inductor, as well as any load transient specifications that may exist.

The output voltage ripple depends directly on the ripple current and is affected by two parameters from the

output capacitor: total capacitance and the capacitors equivalent series resistance (ESR). The output ripple

voltage (worst case) can be found from:

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