Datasheet

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SLVS519A − MAY 2004 − REVISED OCTOBER 2004
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18
APPLICATION INFORMATION
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Figure 25. Application Circuit, 12 V to 3.3 V
Figure 25 shows the schematic for a typical TPS54356
application. The TPS54356 can provide up to 3-A output
current at a nominal output voltage of 3.3 V. For proper
thermal performance, the exposed PowerPAD underneath
the device must be soldered down to the printed circuit
board.
DESIGN PROCEDURE
The following design procedure can be used to select
component values for the TPS54356. Alternately, the
SWIFT Designer Software may be used to generate a
complete design. The SWIFT Designer Software uses an
iterative design procedure and accesses a comprehensive
database of components when generating a design. This
section presents a simplified discussion of the design
process.
To begin the design process a few parameters must be
decided upon. The designer needs to know the following:
D Input voltage range
D Output voltage
D Input ripple voltage
D Output ripple voltage
D Output current rating
D Operating frequency
For this design example, use the following as the input
parameters:
DESIGN PARAMETER EXAMPLE VALUE
Input voltage range 6 V to 18 V
Output voltage 3.3 V
Input ripple voltage 300 mV
Output ripple voltage 10 mV
Output current rating 3 A
Operating frequency 500 kHz
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 kHz to 700 kHz can be set. Use equation 9 to
determine the proper value of RT.
RT(kW) 
46000
ƒ
s
(kHz)  35.9
In this example circuit, RT is not connected and the
switching frequency is set at 500 kHz.
(9)