TPS54550_15 Datasheet, PDF (19/32 Pages) Texas Instruments – 4.5-V TO 20-V INPUT, 6-A OUTPUT SYNCHRONOUS PWM SWITCHER WITH INTEGRATED FET (SWIFT™)

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• Input voltage range
• Output voltage
• Input ripple voltage
• Output ripple voltage
• Output current rating
• Operating frequency
For this design example, use the following as the
input parameters:
DESIGN PARAMETER
EXAMPLE VALUE
Input voltage range
6 V to 17 V
Output voltage
3.3 V
Input ripple voltage
300 mV
Output ripple voltage
30 mV
Output current rating
5A
Operating frequency
700 kHz
NOTE: As an additional constraint, the design is set up to be
small size and low component height.
SWITCHING FREQUENCY
The switching frequency is set using the RT pin.
Grounding the RT pin sets the PWM switching
frequency to a default frequency of 250 kHz. Floating
the RT pin sets the PWM switching frequency to 500
kHz. By connecting a resistor from RT to AGND, any
frequency in the range of 250 to 700 kHz can be set.
Use Equation 8 to determine the proper value of RT.
RT(kW)
+
46000
ƒ_s(kHz) *
35.9
(8)
In this example circuit, the desired switching
frequency is 700 kHz and RT is 69.8 kΩ.
INPUT CAPACITORS
The TPS54550 requires an input decoupling
capacitor and, depending on the application, a bulk
input capacitor. The minimum recommended value
for the decoupling capacitor, C9, is 10 μF. A
high-quality ceramic type X5R or X7R is
recommended. The voltage rating should be greater
than the maximum input voltage. A smaller value
may be used as long as all other requirements are
met; however 10 μF has been shown to work well in
a wide variety of circuits. Additionally, some bulk
capacitance may be needed, especially if the
TPS54550 circuit is not located within about 2 inches
from the input voltage source. The value for this
capacitor is not critical but should be rated to handle
the maximum input voltage including ripple voltage,
and should filter the output so that input ripple
voltage is acceptable.
This input ripple voltage can be approximated by
Equation 9:
TPS54550
SLVS623A – MARCH 2006 – REVISED APRIL 2006
ǒ Ǔ ^{IOUT(MAX) 0.25}
DVIN + _{CBULK ƒsw} ) IOUT(MAX)
ESRMAX
(9)
Where I_OUT(MAX) is the maximum load current, f_SW is
the switching frequency, C_BULK is the bulk capacitor
value and ESR_MAX is the maximum series resistance
of the bulk capacitor.
The maximum RMS ripple current also needs to be
checked. For worst case conditions, this can be
approximated by Equation 10:
ICIN
+
IOUT(MAX)
2
(10)
In this case, the input ripple voltage would be 140
mV and the RMS ripple current would be 2.5 A. It is
also important to note that the actual input voltage
ripple will be greatly affected by parasitics associated
with the layout and the output impedance of the
voltage source. The actual input voltage ripple for
this circuit is shown in Figure 34 and is larger than
the calculated value. This measured value is still
below the specified input limit of 300 mV. The
maximum voltage across the input capacitors would
be VIN max plus Δ 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 very important that
the maximum ratings for voltage and current are not
exceeded under any circumstance.
OUTPUT FILTER COMPONENTS
Two components need to be selected for the output
filter, L1 and C2. Since the TPS54550 is an
externally compensated device, a wide range of filter
component types and values can be supported.
Inductor Selection
To calculate the minimum value of the output
inductor, use Equation 11:
ǒ Ǔ VOUT(MAX) VIN(MAX) * VOUT
LMIN + _{VIN(max) KIND IOUT FSW (11)}
K_IND is a coefficient that represents the amount of
inductor ripple current relative to the maximum
output current. In general, this value is at the
discretion of the designer; however, the following
guidelines may be used. For designs using low ESR
output capacitors such as ceramics, a value as high
as K_IND = 0.3 may be used. When using higher ESR
output capacitors, K_IND = 0.2 yields better results.
For this design example, use K_IND = 0.3 and the
minimum inductor value is calculated to be 3 μH. For
this design, a large value was chosen: 6.8 μH.
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