LTM8045 Datasheet, PDF (15/22 Pages) Linear Technology – Inverting or SEPIC μModule DC/DC Converter with up to 700mA Output Current

LTM8045
APPLICATIONS INFORMATION
Thermal Shutdown
If the part is too hot, the LTM8045 engages its thermal
shutdown, terminates switching and discharges the soft-
start capacitor. When the part has cooled, the part automati-
cally restarts. This thermal shutdown is set to engage at
temperatures above the 125°C absolute maximum internal
operating rating to ensure that it does not interfere with
functionality in the specified operating range. This means
that internal temperatures will exceed the 125°C absolute
maximum rating when the overtemperature protection is
active, possibly impairing the device’s reliability.
PCB Layout
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8045. The LTM8045 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 2
for the suggested layout of the inverting topology applica-
tion and Figure 3 for the suggested layout of the SEPIC
topology application. Ensure that the grounding and heat
sinking are acceptable.
A few rules to keep in mind are:
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 LTM8045.
3. Place the Cout capacitor as close as possible to the
V_OUT+ and V_OUT– connections of the LTM8045.
4. Place the C_IN and C_OUT capacitors such that their
ground currents flow directly adjacent or underneath
the LTM8045.
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 LTM8045.
6. Use vias to connect the GND copper 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 Figures 2 and 3. The LTM8045 can
benefit from the heat sinking afforded by vias that con-
nect to internal GND planes at these locations, due to
their proximity to internal power handling components.
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_OUT–
GND
C_IN
V_IN
R_FB
FB
RUN
C_OUT
RT
GND
R_T GND
GROUND, THERMAL VIAS
8045 F02
Figure 2. Layout Showing Suggested External
Components, GND Plane and Thermal Vias for
the Inverting Topology Application
GND
C_IN
V_IN
FB
RUN
C_OUT
R_FB
RT
V_OUT+
R_T GND
GROUND, THERMAL VIAS
8045 F03
Figure 3. Layout Showing Suggested External
Components, GND Plane and Thermal Vias
for the SEPIC Topology Application
For more information www.linear.com/8045
8045fa
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