Datasheet

I
L(RMS)
+ I
2
OUT(MAX)
)
1
12
ǒ
V
OUT
ǒ
V
IN(MAX)
* V
OUT
Ǔ
V
IN(MAX)
L
OUT
F
SW(MIN)
Ǔ
2
Ǹ
I
L(PK)
+ I
OUT(MAX)
)
V
OUT
ǒ
V
IN(MAX)
* V
OUT
Ǔ
1.6 V
IN(MAX)
L
OUT
F
SW(MIN)
f
CO
+
f
LC
2
85 V
OUT
C
OUT
+
1
3357 L
OUT
f
CO
V
OUT
ESR
MAX
+
1
2p C
OUT
f
CO
V (MAX)=
PP
()
ESR xV xV -V
MAX OUT IN(MAX) OUT
N xV xL
C IN(MAX) OUT
xF
SW
TPS5450-Q1
SLVS834A –JULY 2008– REVISED OCTOBER 2011 www.ti.com
For the output filter inductor it is important that the RMS current and saturation current ratings not be exceeded.
The RMS inductor current can be found from Equation 5:
(5)
and the peak inductor current can be determined with Equation 6:
(6)
For this design, the RMS inductor current is 5.004 A, and the peak inductor current is 5.34 A. The chosen
inductor is a Sumida CDRH1127/LD-150 15μH. It has a minimum rated current of 5.65 A for both saturation and
RMS current. In general, inductor values for use with the TPS5450 are in the range of 10 μH to 100 μH.
Capacitor Selection
The important design factors for the output capacitor are dc voltage rating, ripple current rating, and equivalent
series resistance (ESR). The dc voltage and ripple current ratings cannot be exceeded. The ESR is important
because, along with the inductor ripple current, it determines the amount of output ripple voltage. The actual
value of the output capacitor is not critical, but some practical limits do exist. Consider the relationship between
the desired closed loop crossover frequency of the design and LC corner frequency of the output filter. Due to
the design of the internal compensation, it is desirable to keep the closed loop crossover frequency in the range
3 kHz to 30 kHz, as this frequency range has adequate phase boost to allow for stable operation. For this design
example, it is assumed that the intended closed loop crossover frequency is between 2590 Hz and 24 kHz and
also below the ESR zero of the output capacitor. Under these conditions the closed loop crossover frequency is
related to the LC corner frequency by:
(7)
And the desired output capacitor value for the output filter to:
(8)
For a desired crossover of 12 kHz and a 15-μH inductor, the calculated value for the output capacitor is 330 μF.
The capacitor type should be chosen so that the ESR zero is above the loop crossover. The maximum ESR
should be:
(9)
The maximum ESR of the output capacitor also determines the amount of output ripple as specified in the initial
design parameters. The output ripple voltage is the inductor ripple current times the ESR of the output filter.
Check that the maximum specified ESR as listed in the capacitor data sheet results in an acceptable output
ripple voltage:
(10)
Where:
ΔV
PP
is the desired peak-to-peak output ripple.
N
C
is the number of parallel output capacitors.
F
SW
is the switching frequency.
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