lm20133mhx National Semiconductor Corporation, lm20133mhx Datasheet - Page 17

• Manufacturer Part Number: lm20133mhx
• Description: 3a, Powerwise Synchronous Buck Regulator With Input Synchronization
• Manufacturer: National Semiconductor Corporation

T
I
DCR is the inductor series resistance.
It is important to always keep the operating junction temper-
ature (T
temperature exceeds 160°C the device will cycle in and out
of thermal shutdown. If thermal shutdown occurs it is a sign
of inadequate heatsinking or excessive power dissipation in
the device.
Figure 9, shown below, provides a better approximation of the
θ
consists of 2oz. copper located on the bottom layer of the PCB
directly under the eTSSOP exposed pad. The bottom copper
area is connected to the eTSSOP exposed pad by means of
a 4 x 4 array of 12 mil thermal vias.
PCB LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter de-
sign. Poor board layout can disrupt the performance of a DC-
DC converter and surrounding circuitry by contributing to EMI,
ground bounce, and resistive voltage loss in the traces. These
can send erroneous signals to the DC-DC converter resulting
in poor regulation or instability.
Good layout can be implemented by following a few simple
design rules.
1. Minimize area of switched current loops. In a buck regulator
there are two loops where currents are switched very fast. The
first loop starts from the input capacitor, to the regulator VIN
pin, to the regulator SW pin, to the inductor then out to the
OUT
A
JA
is the ambient temperature in °C.
for a given PCB copper area. The PCB heatsink area
is the output load current.
FIGURE 9. Thermal Resistance vs PCB Area
J
) below 125°C for reliable operation. If the junction
30030335
17
output capacitor and load. The second loop starts from the
output capacitor ground, to the regulator PGND pins, to the
inductor and then out to the load (see Figure 10). To minimize
both loop areas the input capacitor should be placed as close
as possible to the PVIN pin. Grounding for both the input and
output capacitor should consist of a small localized top side
plane that connects to PGND and the die attach pad (DAP).
The inductor should be placed as close as possible to the SW
pin and output capacitor.
2. Minimize the copper area of the switch node. Since the
LM20133 has the SW pins on opposite sides of the package
it is recommended to via these pins down to the bottom or
internal layer with 2 to 4 vias on each SW pin. The SW pins
should be directly connected with a trace that runs across the
bottom of the package. To minimize IR losses this trace
should be no smaller that 50 mils wide, but no larger than 100
mils wide to keep the copper area to a minimum. In general
the SW pins should not be connected on the top layer since
it could block the ground return path for the power ground.
The inductor should be placed as close as possible to one of
the SW pins to further minimize the copper area of the switch
node.
3. Have a single point ground for all device analog grounds
located under the DAP. The ground connections for the com-
pensation, feedback, and Soft-Start components should be
connected together then routed to the AGND pin of the de-
vice. The AGND pin should connect to PGND under the DAP.
This prevents any switched or load currents from flowing in
the analog ground plane. If not properly handled poor ground-
ing can result in degraded load regulation or erratic switching
behavior.
4. Minimize trace length to the FB pin. Since the feedback
node can be high impedance the trace from the output resistor
divider to FB pin should be as short as possible. This is most
important when high value resistors are used to set the output
voltage. The feedback trace should be routed away from the
SW pin and inductor to avoid contaminating the feedback sig-
nal with switch noise.
5. Make input and output bus connections as wide as possi-
ble. This reduces any voltage drops on the input or output of
the converter and can improve efficiency. If voltage accuracy
at the load is important make sure feedback voltage sense is
made at the load. Doing so will correct for voltage drops at the
load and provide the best output accuracy.
6. Provide adequate device heatsinking. Use as many vias as
is possible to connect the DAP to the power plane heatsink.
For best results use a 4x4 via array with a minimum via di-
ameter of 12 mils. See the Thermal Considerations section to
insure enough copper heatsinking area is used to keep the
junction temperature below 125°C.
www.national.com