Datasheet

( )
´
´
´
D +
0.25
OUT(MAX)
=
IN
MAX
OUT(MAX)
BULK
SW
I
V I ESR
C ¦
OUT(MAX)
CIN
I
I
2
=
( )
F
)
IN(MAX
OUT OUT
MIN
IND
OUT
IN(MAX) SW
V V -V
L
V K I
´
=
´ ´ ´
TPS54110
www.ti.com 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, 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:
(3)
In this case the input ripple voltage is 66 mV with a 10-µF 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
+
ΔV
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:
(4)
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 µH. The next-higher standard
value of 6.8 µH 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:
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