Datasheet

I
L(PK)
=I +
OUT(MAX)
()
V V V
OUT IN(MAX) OUT
x -
1.6 xV xL xF
IN(MAX) OUT SW
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
L
MIN
=
()
V
OUT IN(MAX) OUT
xV -V
V
IN(max) IND OUT SW
xK xI xF x0.8
TPS5420
SLVS642E –APRIL 2006–REVISED SEPTEMBER 2013 www.ti.com
In this example, the calculated input ripple voltage is 118 mV, and the RMS ripple current is 1.0 A. The maximum
voltage across the input capacitors would be VIN max plus delta VIN/2. The chosen input decoupling capacitors
are rated for 50 V, and the ripple current capacity for each is 3 A at 500 kHz, providing ample margin. The actual
measured input ripple voltage may be larger than the calculated value due to the output impedance of the input
voltage source and parasitics associated with the layout.
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 2 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:
(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 is 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 27 μH. The standard value used in
this design is 33 μH.
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 using Equation 6:
(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 μH. It has a saturation current rating of
2.2 A and a RMS current rating of 2.7 A, which meets the requirements. Inductor values for use with the
TPS5420 are in the range of 10 μH to 100 μH.
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