
(
)
D
+
0.25
OUT(MAX)
=
IN
MAX
OUT(MAX)
BULK
SW
I
V
I
ESR
C
OUT(MAX)
CIN
I
2
=
(
)
F
)
IN(MAX
OUT
MIN
IND
OUT
IN(MAX)
SW
V
-V
L
V
K
I
=
SLVS500C
– DECEMBER 2003 – REVISED FEBRUARY 2011
Alternately, the TPS54110 can be set to preprogrammed switching frequencies of 350 kHz or 550 kHz by
connecting pins RT and SYNC as shown in
Table 2.Table 2. Design Parameters
FREQUENCY
RT
SYNC
350 kHz
Float
Float or AGND
550 kHz
Float
≥ 2.5 V
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 uF. 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:(2)
Where
IOUT(MAX) is the maximum load current,
SW is the switching frequency, CBULK is the bulk capacitor value and
ESRMAX is the maximum series resistance of the bulk capacitor.
Worst-case RMS ripple current is approximated by
Equation 3:(3)
In this case the input ripple voltage is 66 mV with a 10-
F 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 VINMAX +
ΔVIN/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:
(4)
KIND 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 KIND = 0.2. When using higher ESR output
capacitors, KIND = 0.1 yields better results. If higher ripple currents can be tolerated, KIND can be increased
allowing for a smaller output-inductor value.
This example design uses KIND = 0.2, yielding a minimum inductor value of 6.29 H. The next-higher standard
value of 6.8
H 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
2003–2011, Texas Instruments Incorporated
9