LTM8029_1208 LINER [Linear Technology], LTM8029_1208 Datasheet - Page 14

• Manufacturer Part Number: LTM8029_1208
• Description: 36VIN 600mA Step-Down ?Module Converter with 5?A Quiescent Current
• Manufacturer: LINER [Linear Technology]

LTM8029
applicaTions inForMaTion
Power Good
The PGOOD pin is the open-collector output of an internal
comparator that monitors the voltage at the FB pin. It
is used to indicate whether the output is near or within
regulation. Specifically, PGOOD is low unless the FB pin is
within 10% of the final regulation voltage. PGOOD output
is valid when V
function is not used, leave this pin floating.
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8029. However, these capacitors
can cause problems if the LTM8029 is hot-plugged into
a live supply (see Application Note 88 for a complete dis-
cussion). The low loss ceramic capacitor combined with
stray inductance in series with the power source forms an
underdamped tank circuit, and the voltage at the V
of the LTM8029 can ring to more than twice the nominal
input voltage, possibly exceeding the LTM8029’s rating
and damaging the part. If the input supply is poorly con-
trolled or the user will be hot-plugging the LTM8029 into
an energized supply, the input network should be designed
to prevent this overshoot. This can be accomplished by
installing a small resistor in series to V
popular method of controlling input voltage overshoot is
to add an electrolytic bulk capacitor to the V
capacitor’s relatively high equivalent series resistance
usually damps the circuit and eliminates the voltage
overshoot. The extra capacitor improves low frequency
ripple filtering and can slightly improve the efficiency of
the circuit, though it is likely to be the largest component
in the circuit.
PCB Layout
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8029. The LTM8029 is neverthe-
less a switching power supply, and care must be taken to
minimize EMI and ensure proper operation. Even with the
high level of integration, you may fail to achieve specified
operation with a haphazard or poor layout. See Figure 3
for a suggested layout. Ensure that the grounding and
heat sinking are acceptable.
14
IN
is above 4.5V and RUN is high. If this
IN
, but the most
IN
net. This
IN
pin
1. Place the R
2. Place the C
3. Place the C
4. Place the C
5. Connect all of the GND connections to as large a copper
6. For good heat sinking, use vias to connect the GND cop-
their respective pins.
and GND connection of the LTM8029.
V
ground currents flow directly adjacent or underneath
the LTM8029.
pour or plane area as possible on the top layer. Avoid
breaking the ground connection between the external
components and the LTM8029.
per area to the board’s internal ground planes. Liberally
distribute these GND vias to provide both a good ground
connection and thermal path to the internal planes of the
printed circuit board. Pay attention to the location and
density of the thermal vias in Figure 3. The LTM8029
can benefit from the heat sinking afforded by vias that
connect to internal GND planes at these locations, due
to their proximity to internal power handling compo-
nents. The optimum number of thermal vias depends
upon the printed circuit board design. For example, a
board might use very small via holes. It should employ
more thermal vias than a board that uses larger holes.
OUT
and GND connection of the LTM8029.
Figure 3. Layout Showing Suggested External
Components, GND Plane and Thermal Vias
V
RUN
IN
R
IN
FB
OUT
FB
IN
capacitor as close as possible to the V
PGOOD
and R
and C
capacitor as close as possible to the
C
IN
R
T
T
resistors as close as possible to
OUT
GND
THERMAL VIAS
capacitors such that their
BIAS
V
OUT
C
OUT
GND
8029 F03
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IN