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
O
O IN SW
V max V V
L min
Kind I V max f
æ ö
-
³ ´
ç ÷
´ ´
è ø
TPS54062
www.ti.com SLVSAV1B –MAY 2011–REVISED AUGUST 2012
Selecting the Switching Frequency
The first step is to decide on a switching frequency for the regulator. Typically, the user will want to choose the
highest switching frequency possible since this will produce the smallest solution size. The high switching
frequency allows for lower valued inductors and smaller output capacitors compared to a power supply that
switches at a lower frequency. The switching frequency that can be selected is limited by the minimum on-time of
the internal power switch, the input voltage and the output voltage and the frequency shift limitation.
Equation 5 and Equation 6 must be used to find the maximum switching frequency for the regulator, choose the
lower value of the two equations. Switching frequencies higher than these values will result in pulse skipping or
the lack of overcurrent protection during a short circuit. The typical minimum on time, t
on
min, is 130ns for the
TPS54062. For this example, the output voltage is 3.3V and the maximum input voltage is 60 V, which allows for
a maximum switch frequency up to 400 kHz when including the inductor resistance, on resistance and diode
voltage in Equation 5 or Equation 6. To ensure overcurrent runaway is not a concern during short circuits in your
design use Equation 6 to determine the maximum switching frequency. With a maximum input voltage of 60V,
inductor resistance of 3.7 Ω, high side switch resistance of 2.3 Ω, low side switch resistance of 1.1Ω, a current
limit value of 120 mA and a short circuit output voltage of 0.1 V.
The maximum switching frequency is 400 kHz in both cases and a switching frequency of 400 kHz is used. To
determine the timing resistance for a given switching frequency, use Equation 4. The switching frequency is set
by resistor R3 shown in Figure 20. R3 is calculated to be 298 kΩ. A standard value of 301 kΩ is used.
Output Inductor Selection (LO)
To calculate the minimum value of the output inductor, use Equation 7. 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 value as high as KIND = 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 = 0.8 and the minimum inductor value is calculated to be 195 µH. For this design, a
near standard value was chosen: 220 µH. For the output filter inductor, it is important that the RMS current and
saturation current ratings not be exceeded. The RMS and peak inductor current can be found from Equation 9
and Equation 10.
For this design, the RMS inductor current is 50 mA and the peak inductor current is 68 mA. The chosen inductor
is a Coilcraft LPS4018-224ML. It has a saturation current rating of 235 mA and an RMS current rating of 200 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 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.
(7)
(8)
(9)
(10)
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