Datasheet

L
MIN
=
()
V
OUT IN(MAX) OUT
×V – V
V
IN(max) IND OUT SW
×K ×I ×F ×0.8
(4)
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
0.8
Ǔ
2
Ǹ
(5)
I
L(PK)
=I +
OUT(MAX)
()
V V V
OUT IN(MAX) OUT
× –
1.6 ×V ×L ×F
IN(MAX) OUT SW
(6)
TPS5420-Q1
SLVS752B – NOVEMBER 2007 – REVISED JUNE 2008 ................................................................................................................................................... www.ti.com
CAUTION:
The maximum ratings for voltage and current are not to be exceeded under any
circumstance.
Additionally, some bulk capacitance may be needed, especially if the TPS5420 circuit is not located within
approximately two inches from the input voltage source. The value for this capacitor is not critical, but it should
be rated to handle the maximum input voltage including ripple voltage and should filter the output so that input
ripple voltage is acceptable.
Output Filter Components
Two components need to be selected for the output filter, L1 and C2. Since the TPS5420 is an internally
compensated device, a limited range of filter component types and values can be supported.
Inductor Selection
To calculate the minimum value of the output inductor, use Equation 4 :
K
IND
is a coefficient that represents the amount of inductor ripple current relative to the maximum output current.
Three things need to be considered when determining the amount of ripple current in the inductor: the
peak-to-peak ripple current affects the output ripple voltage amplitude, the ripple current affects the peak switch
current, and the amount of ripple current determines at what point the circuit becomes discontinuous. For
designs using the TPS5420, K
IND
of 0.2 to 0.3 yields good results. Low output ripple voltages are obtained when
paired with the proper output capacitor, the peak switch current is below the current limit set point, and low load
currents can be sourced before discontinuous operation.
For this design example, use K
IND
= 0.2, and the minimum inductor value is 31 µ H. The next highest standard
value used in this design is 33 µ H.
For the output filter inductor, it is important that the RMS current and saturation current ratings not be exceeded.
The RMS inductor current is found from Equation 5 :
and the peak inductor current is determined from Equation 6 :
For this design, the RMS inductor current is 2.002 A, and the peak inductor current is 2.16 A. The chosen
inductor is a Coilcraft MSS1260-333 type. The nominal inductance is 33 µ H. It has a saturation current rating of
2.2 A and a RMS current rating of 2.7 A, which meet the requirements. Inductor values for use with the TPS5420
are in the range of 10 µ H to 100 µ H.
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
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