LTM4613 datasheet(15/22 Pages) LINER

15 / 22 page

LTM8045

8045fa

For more information www.linear.com/8045

APPLICATIONS INFORMATION

Thermal Shutdown

If the part is too hot, the LTM8045 engages its thermal

shutdown, terminates switching and discharges the soft-

startcapacitor.Whentheparthascooled,thepartautomati-

cally restarts. This thermal shutdown is set to engage at

temperaturesabovethe125°Cabsolutemaximuminternal

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 RFB and RT resistors as close as possible to

their respective pins.

2. Place the CIN capacitor as close as possible to the VIN

and GND connection of the LTM8045.

3. Place the Cout capacitor as close as possible to the

VOUT+ and VOUT– connections of the LTM8045.

4. Place the CIN and COUT 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.

Figure 2. Layout Showing Suggested External

Components, GND Plane and Thermal Vias for

the Inverting Topology Application

8045 F02

GROUND, THERMAL VIAS

VIN

CIN

RFB

VOUT–

GND

RUN

GND

COUT

Figure 3. Layout Showing Suggested External

Components, GND Plane and Thermal Vias

for the SEPIC Topology Application

8045 F03

GROUND, THERMAL VIAS

VIN

CIN

RFB

VOUT+

GND

COUT

RUN

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