TPS40210-Q1, TPS40211-Q1
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SLVS861D –AUGUST 2008–REVISED APRIL 2010
To set the mid-band gain of the error amplifier to KCOMP use Equation 63.
R7
51.1kW
R4 = R7´KCOMP
=
=
= 18.2kW
KCO
2.80
(63)
R4 = 18.7 kΩ selected.
Place the zero at 10th the desired cross-over frequency.
10 10
2p´ f ´R4 2p´ 30kHz ´18.7kW
C2 =
=
= 2837pF
L
(64)
C2 = 2200 pF selected.
Place a high-frequency pole at about five times the desired cross-over frequency and less than one-half the unity
gain bandwidth of the error amplifier:
1
1
C4 »
=
10p´ f ´R4 10p´30kHz ´18.7kW
= 56.74pF
L
(65)
(66)
1
1
C4 >
=
p´ GBW ´R4 p´1.5MHz ´18.7kW
= 11.35pF
C4 = 47 pF selected.
R-C Oscillator
The R-C oscillator calculation as shown in Equation 5 substitutes 100 for CT and 600 for fSW. For a 600-kHz
switching frequency, a 100-pF capacitor is selected and a 262-kΩ resistor is calculated (261 kΩ selected).
Soft-Start Capacitor
Because VDD > 8 V, the soft-start capacitor is selected by using Equation 67 to calculate the value.
-6
C
= 20´ T ´10
SS
SS
(67)
For TSS = 12 ms, CSS = 240 nF, a 220-nF capacitor selected.
Regulator Bypass
A regulator bypass capacitor of 1.0-mF is selected per the recommendation.
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