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I
CIN +
I
OUT(MAX)
2
(11)
OUTPUT FILTER COMPONENTS
Inductor Selection
L
MIN +
V
OUT(MAX)
V
IN(MAX) *
V
OUT
V
IN(max)
K
IND
I
OUT
F
SW
(12)
I
L(RMS)
+
I2
OUT(MAX) )
1
12
V
OUT
V
IN(MAX)
* VOUT
V
IN(MAX)
L
OUT
F
SW
0.8
2
(13)
I
L(PK) +
I
OUT(MAX) )
V
OUT
V
IN(MAX) *
V
OUT
1.6
V
IN(MAX)
L
OUT
F
SW
(14)
Capacitor Selection
TPS54350-EP
SGLS308 – OCTOBER 2005
In this case the input ripple voltage would be 140 mV and the RMS ripple current would be 1.5 A. The maximum
voltage across the input capacitors would be VIN max plus delta VIN/2. The chosen bulk and bypass capacitors
are each rated for 25 V and the combined ripple current capacity is greater than 3 A, both providing ample
margin. It is important that the maximum ratings for voltage and current are not exceeded under any
circumstance.
Two components need to be selected for the output filter, L1 and C2. Since the TPS54350 is an externally
compensated device, a wide range of filter component types and values can be supported.
To calculate the minimum value of the output inductor, use Equation 12:
KIND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current.
For designs using low ESR output capacitors such as ceramics, use KIND = 0.3. When using higher ESR output
capacitors, KIND = 0.2 yields better results.
For this design example, use KIND = 0.2 and the minimum inductor value is calculated to be 8.98 H. The next
highest standard value is 10 H, which is used in this design.
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 13:
and the peak inductor current can be determined with Equation 14:
For this design, the RMS inductor current is 3.01 A and the peak inductor current is 3.34 A. The chosen inductor
is a Vishay IHLP5050CE-01 10 H. It has a saturation current rating of 14 A and a RMS current rating of 7 A,
easily meeting these requirements. A lesser rated inductor could be used, however this device was chosen
because of its low profile component height. In general, inductor values for use with the TPS54350 are in the
range of 6.8 H to 47mH.
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 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. In general, it is
desirable to keep the closed loop crossover frequency at less than 1/5 of the switching frequency. With high
switching frequencies such as the 500-kHz frequency of this design, internal circuit limitations of the TPS54350
limit the practical maximum crossover frequency to about 50 kHz. Additionally, to allow for adequate phase gain
in the compensation network, the LC corner frequency should be about one decade or so below the closed loop
crossover frequency. This limits the minimum capacitor value for the output filter to:
20