LTM8029_1208 Datasheet, PDF (14/20 Pages) Linear Technology – 36VIN, 600mA Step-Down Module Converter with 5A Quiescent Current

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_IN is above 4.5V and RUN is high. If this
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_IN pin
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_IN, but the most
popular method of controlling input voltage overshoot is
to add an electrolytic bulk capacitor to the V_IN net. This
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
1. Place the R_FB and R_T resistors as close as possible to
their respective pins.
2. Place the C_IN capacitor as close as possible to the V_IN
and GND connection of the LTM8029.
3. Place the C_OUT capacitor as close as possible to the
V_OUT and GND connection of the LTM8029.
4. Place the C_IN and C_OUT capacitors such that their
ground currents flow directly adjacent or underneath
the LTM8029.
5. Connect all of the GND connections to as large a copper
pour or plane area as possible on the top layer. Avoid
breaking the ground connection between the external
components and the LTM8029.
6. For good heat sinking, use vias to connect the GND cop-
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.
V_IN
C_IN
GND
V_OUT
RUN
PGOOD
BIAS
C_OUT
GND
R_FB
R_T
THERMAL VIAS
8029 F03
Figure 3. Layout Showing Suggested External
Components, GND Plane and Thermal Vias
8029fa