Datasheet

TPS54110−Q1
SLVS837 − JULY 2008
www.ti.com
9
INPUT CAPACITORS
The TPS54110 requires an input decoupling capacitor
and, depending on the application, a bulk input capacitor.
The minimum value for the decoupling capacitor, C9, is
10 µF. A high-quality ceramic type X5R or X7R with a
voltage rating greater than the maximum input voltage is
recommended. A bulk input capacitor may be needed,
especially if the TPS54110 circuit is not located within
approximately 2 inches from the input voltage source. The
capacitance value is not critical, but the voltage rating must
be greater than the maximum input voltage including ripple
voltage. The capacitor must filter the input ripple voltage to
acceptable levels.
Input ripple voltage can be approximated by equation 2:
DV
IN
+
I
OUT(MAX)
0.25
C
BULK
ƒ
sw
)
ǒ
I
OUT(MAX)
ESR
MAX
Ǔ
Where
IOUT(MAX) is the maximum load current,
ƒ
SW
is the switching frequency,
C
BULK
is the bulk capacitor value and
ESR
MAX
is the maximum series resistance of the
bulk capacitor.
Worst-case RMS ripple current is approximated by
equation 3:
I
CIN
+
I
OUT(MAX)
2
In this case the input ripple voltage is 66 mV with a 10-uF
bulk capacitor. Figure 15 shows the measured ripple
waveform. The RMS ripple current is 0.75 A. The
maximum voltage across the input capacitors is V
INMAX
+
∆V
IN
/2. The bypass capacitor and input bulk capacitor are
each rated for 6.3 V and a ripple-current capacity of 1.5 A,
providing some margin. It is very important that the
maximum ratings for voltage and current are not exceeded
under any circumstance.
OUTPUT FILTER COMPONENTS
Two components, L1 and C2, are selected for the output
filter. Since the TPS54110 is an externally-compensated
device, a wide range of filter-component types and values
are supported.
Inductor Selection
Use equation 4 to calculate the minimum value of the
output inductor:
L
MIN
+
V
OUT
ǒ
V
IN(MAX)
* V
OUT
Ǔ
V
IN(MAX)
K
IND
I
OUT
F
SW
K
IND
is a coefficient that represents the amount of inductor
ripple current relative to the maximum output current. For
designs using low-ESR capacitors such as ceramics, use
K
IND
= 0.2. When using higher ESR output capacitors,
K
IND
= 0.1 yields better results. If higher ripple currents can
be tolerated, K
IND
can be increased allowing for a smaller
output-inductor value.
This example design uses K
IND
= 0.2, yielding a minimum
inductor value of 6.29 uH. The next-higher standard value
of 6.8 uH is chosen for this design. If a lower inductor value
is desired, a larger amount of ripple current must be
tolerated.
The RMS-current and saturation-current ratings of the
output filter inductor must not be exceeded. The RMS
inductor current can be found from equation 5:
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
The peak inductor current is determined from equation 6:
I
L(PK)
+ I
OUT(MAX)
)
V
OUT
ǒ
V
IN(MAX)
* V
OUT
Ǔ
1.6 V
IN(MAX)
L
OUT
F
SW
For this design, the RMS inductor current is 1.503 A and
the peak inductor current is 1.673 A. The inductor chosen
is a Coilcraft DS3316P-682 6.8 µH. It has a saturation-
current rating of 2.8 A and an RMS current rating of 2.2 A,
easily meeting these requirements.
Capacitor Selection
The important design parameters for the output capacitor
are dc voltage, ripple current, and equivalent series
resistance (ESR). The dc-voltage and ripple-current
ratings must not be exceeded. The ESR rating is important
because along with the inductor current it determines the
output ripple voltage level. 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
one-fifth of the switching frequency. With high switching
frequencies such as the 700 kHz frequency of this design,
internal circuit limitations of the TPS54110 limit the
practical maximum crossover frequency to about 100 kHz.
To allow adequate phase gain in the compensation
network, set the LC corner frequency to approximately one
decade below the closed-loop crossover frequency. This
limits the minimum capacitor value for the output filter to:
C
OUT(MIN)
+
1
L
OUT
ǒ
K
2pƒ
CO
Ǔ
2
(2)
(3)
(4)
(5)
(6)
(7)