Datasheet

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f − Switching Frequency − kHz
R
T
− Timing Resistance − kΩ
100
300
100
150 200 250 350300
350
200
450
550
400
250
500
150
f
SW
− Switching Frequency − kHz
R
T
− Timing Resistance − kΩ
250
100
50
350300 400 500450 600550
125
75
225
175
200
150
Selecting an Inductor Value
L +
ǒ
V
IN
* V
OUT
Ǔ
V
OUT
V
IN
f
SW
DI
(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)
• V
IN
is the input voltage to the converter (V)
• V
OUT
is the output voltage of the converter (V)
• f
SW
is the switching frequency chosen for the converter (Hz)
•∆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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