TPS54873

SLVS444 – OCTOBER 2002

www.ti.com

9

OPERATING FREQUENCY

In the application circuit, the RT pin is grounded through a

71.5-k

Ω resistor (R6) to select the operating frequency of

700 kHz. To set a different frequency, place a 68-k

Ω to

180-k

Ω resistor between RT (pin 28) and analog ground or

leave RT floating to select the default of 350 kHz. The

resistance can be approximated using the following

equation:

R

+

500 kHz

Switching Frequency

100 [k

W]

OUTPUT FILTER

The output filter is composed of a 0.65-

µH inductor (L1)

and 3 x 22-

µF capacitors (C5, C7 and C8). The inductor is

a low dc resistance (.017

Ω) type, Pulse PA0277 0.65 µH.

The capacitors used are 22-

µF, 6.3-V ceramic types with

X5R dielectric. An additional high frequency bypass

capacitor, C13 is also used.

PRECHARGE CIRCUIT

VIN precharges the output of the application circuit

through series diodes (D1 and D2) during start-up. As the

input voltage increases at start-up, the output is

precharged to VIN minus the forward bias voltage of the

two diodes. When the internal reference has ramped up to

a value greater than the voltage fed back to the VSENSE

pin, the output of the internal error amplifier begins to

increase. When this output reaches the maximum ramp

amplitude, the output of the PWM comparator reaches 100

percent duty cycle and the internal logic enables the

high-side FET driver and switching begins. The output

tracks the internal reference until the preset output voltage

is reached. Under no circumstances should the precharge

voltage be allowed to increase above the preset output

value.

GROUNDING AND POWERPAD LAYOUT

The TPS54873 has two internal grounds (analog and

power). Inside the TPS54873, the analog ground ties to all

of the noise sensitive signals, while the power ground ties

to the noisier power signals. The PowerPAD must be tied

directly to AGND. Noise injected between the two grounds

can degrade the performance of the TPS54873,

particularly at higher output currents. However, ground

noise on an analog ground plane can also cause problems

with some of the control and bias signals. For these

reasons, separate analog and power ground areas are

recommended. The analog ground area should be tied to

the power ground area directly at the IC to reduce noise

between the two grounds. The only components that

should tie directly to the power ground area are the input

capacitor, the output capacitor, the input voltage

decoupling capacitor, and the PGND pins of the

TPS54873. The power ground areas as well as the

PowerPAD mounting area should be tied to any internal

ground planes using multiple vias. The layout of the

TPS54873 evaluation module is representative of a

recommended layout for a 4-layer board with the two

internal layers representing the system ground plane.

Documentation for the TPS54873 evaluation module can

be found on the Texas Instruments web site under the

TPS54873 product folder.

LAYOUT CONSIDERATIONS FOR THERMAL

PERFORMANCE

For operation at full rated load current, the analog ground

plane must provide adequate heat dissipating area. A 3

inch by 3 inch plane of 1 ounce copper is recommended,

though not mandatory, depending on ambient temperature

and airflow. Most applications have larger areas of internal

ground plane available, and the PowerPAD should be

connected to the largest area available. Additional areas

on the top or bottom layers also help dissipate heat, and

any area available should be used when 6 A or greater

operation is desired. Connection from the exposed area of

the PowerPAD to the analog ground plane layer should be

made using 0.013 inch diameter vias to avoid solder

wicking through the vias. Eight vias should be in the

PowerPAD area with four additional vias located under the

device package. The size of the vias under the package,

but not in the exposed thermal pad area, can be increased

to 0.018. Additional vias beyond the ten recommended

that enhance thermal performance should be included in

areas not under the device package.

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