Datasheet

RIPPLE
L OUT
I
I peak = I +
2
( )
2
OUT IN OUT
2
L O
IN O SW
V V max V
1
I rms = I +
12 V max L f
æ ö
´ -
´
ç ÷
ç ÷
´ ´
è ø
( )
OUT IN OUT
RIPPLE
IN O SW
V V max V
I
V max L f
´ -
³
´ ´
IN OUT OUT
IN sw
V max V V
L min
O
Kind I V max
O
-
³ ´
´ ´ ¦
TPS54061
SLVSBB7C –MAY 2012–REVISED JANUARY 2014 www.ti.com
Output Inductor Selection (LO)
To calculate the minimum value of the output inductor, use Equation 8. KIND is a coefficient that represents the
amount of inductor ripple current relative to the maximum output current. The inductor ripple current will be
filtered by the output capacitor. Therefore, choosing high inductor ripple currents will impact 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, the following
guidelines may be used. Typically it is recommended to use KIND values in the range of 0.2 to 0.4; however, for
designs using low ESR output capacitors such as ceramics and low output currents, a KIND value as high as 1
may be used. In a wide input voltage regulator, it is best to choose an inductor ripple current on the larger side.
This allows the inductor to still have a measurable ripple current with the input voltage at its minimum. For this
design example, use KIND of 0.4 and the minimum inductor value is calculated to be 97 µH. For this design, a
standard 100µH value was chosen. It is important that the RMS current and saturation current ratings of the
inductor not be exceeded. The RMS and peak inductor current can be found from Equation 10 and Equation 11.
For this design, the RMS inductor current is 200 mA and the peak inductor current is 239 mA. The chosen
inductor is a Würth 74408943101. It has a saturation current rating of 680 mA and an RMS current rating of 520
mA. As the equation set demonstrates, lower ripple currents will reduce the output voltage ripple of the regulator
but will require a larger value of inductance. Selecting higher ripple currents will increase the output voltage ripple
of the regulator but allow for a lower inductance value. The current flowing through the inductor is the inductor
ripple current plus the average output current. During power up, faults or transient load conditions, the inductor
current can increase above the 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 calculated peak inductor current.
(8)
(9)
(10)
(11)
Output Capacitor
There are three primary considerations for selecting the value of the output capacitor. The output capacitor will
determine the modulator pole, the output voltage ripple, and how the regulator responds to a large change in
load current. The output capacitance needs to be selected based on the most stringent of these three criteria.
The desired response to a large change in the load current is the first criteria. The output capacitor needs to
supply the load with current until the regulator increases the inductor current. 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 also will temporarily
not be 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 a full load. The regulator usually needs two or more clock cycles for the
control loop to see the change in load current and output voltage and adjust the duty cycle to react to the
change. The output capacitor must be sized to supply the extra current to the load until the control loop responds
to the load change. The output capacitance must be large enough to supply the difference in current for 2 clock
cycles while only allowing a tolerable amount of droop in the output voltage. Equation 15 shows the minimum
output capacitance necessary to accomplish this, where ΔIout is the change in output current, ƒ
sw
is the
regulators switching frequency and ΔVout is the allowable change in the output voltage.
For this example, the transient load response is specified as a 4% change in Vout for a load step from 50 mA to
150 mA. For this example, ΔI
OUT
= 0.150 –0.05 = 0.10 and ΔV
OUT
= 0.04 × 3.3 = 0.132.
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