Datasheet

2
= +
Iripple
ILpeak Iout
( )
2
2
1
12
Vout Vinmax Vout
ILrms Iout
Vinmax L1 sw
æ ö
× -
= + ×
ç ÷
ç ÷
× ×
è ø
f
f
-
= ×
×
Vinmax Vout Vout
Iripple
L1 Vinmax sw
Vinmax Vout Vout
L1
Iout Kind Vinmax sw
-
= ×
× × f
TPS54521
www.ti.com SLVS981C –JUNE 2010–REVISED AUGUST 2013
Operating Frequency
The first step is to decide on a switching frequency for the regulator. There is a trade off between higher and
lower switching frequencies. Higher switching frequencies may produce a smaller solution size using lower
valued inductors and smaller output capacitors compared to a power supply that switches at a lower frequency.
However, the higher switching frequency causes additional switching losses, which hurt the converter’s efficiency
and thermal performance. In this design, a moderate switching frequency of 700 kHz is selected to achieve both
a small solution size and a high efficiency operation. This frequency is set using the resistor at the RT/CLK pin
(R3). Using Equation 4, the resistance required for a switching frequency of 700 kHz is 69.9 kΩ. A 69.8 kΩ
resistor is used for this design.
Output Inductor Selection
To calculate the value of the output inductor Equation 18 is used. Kind is a coefficient that represents the amount
of inductor ripple current relative to the maximum output current. The inductor ripple current is filtered by the
output capacitor. Therefore, choosing high inductor ripple currents impacts the selection of the output capacitor,
since the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. In
general, the inductor ripple value is at the discretion of the designer; however, Kind is normally from 0.3 to 0.4 for
the majority of low cost applications.
(18)
For this design example, using Kind = 0.35, the inductor value is calculated to be 2.9 µH. A 3.3 µH inductor from
Coilcraft’s MSS1260 series was chosen. For the output filter inductor, it is important that the RMS current and
saturation current ratings not be exceeded. The inductor ripple current, RMS current, and peak inductor current
can be found from Equation 19, Equation 20, and Equation 21.
(19)
(20)
(21)
For this design, the inductor ripple current is 1.53 A, the RMS inductor current is 5.02 A, and the peak inductor
current is 5.76 A. The chosen inductor has a RMS current rating of 7 A and a saturation current rating of 10.4 A.
The current flowing through the inductor is the inductor ripple current plus the output current. During power up,
faults, or transient load conditions, the inductor current can increase above the calculated peak inductor current
level calculated above. In transient conditions, the inductor current can increase up to the switch current limit of
the device. For this reason, the most conservative approach is to specify an inductor with a saturation current
rating equal to or greater than the switch current limit rather than the peak inductor current.
Output Capacitor Selection
There are two primary considerations for selecting the output capacitor: the minimum capacitance required to
meet the transient response specification and the maximum impedance at the switching frequency to meet the
output voltage ripple requirement. Any output capacitor type (ceramic, tantalum, polymer, electrolytic, etc.) can
be used with the TPS54521 to meet the design specifications.
The desired response to a large change in the load current is the first criterion. The output capacitor needs to
supply the load with current when the regulator cannot. This situation would occur if there are desired hold-up
times for the regulator where the output capacitor must hold the output voltage above a certain level for a
specified amount of time after the input power is removed. The regulator is also temporarily not able to supply
sufficient output current if there is a large, fast increase in the current needs of the load, such as transitioning
from no load to full load. The regulator usually needs two or more clock cycles for the control loop to see the
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