Thermal Guidelines and Layout Recommendations

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4.2

Output Load Transient

Figure 2 shows the V

Figure 2. Step Load and Output Voltage Transient Response

5

Thermal Guidelines and Layout Recommendations

Thermal management is a key consideration in the design of any dc-dc converter but is especially

important for an LDO when the power dissipation is high. Use the equation below to approximate the

worst case junction temperature for the application:

T

(1)

where T

in the device (Watts), and

θ

silicon junction temperature should not be allowed to exceed 125°C for reliable operation. The layout

design must use copper traces and plane areas smartly, as thermal sinks, so as to not allow T

the absolute maximum rating under all load, voltage, and temperature conditions for a given application.

The layout should consider carefully the thermal design of the PCB for optimal performance over

temperature. For this EVM, Figure 4 shows that the RGW package footprint employs a square thermal

pad, centered under the part, for conducting heat to the copper spreading layers of the PCB. The thermal

pad is soldered directly to a pad on the PCB containing a 5 × 5 pattern of 10.mil vias for conducting heat

to the bottom side ground plane copper. Approximately 4.0 in

side of the EVM for dissipating heat generated by the LDO.

Table 2 relies on thermal resistance information from the Thermal Information Table of the TPS7A4700

data sheet for comparison with the approximate thermal resistance,

θ

show the variation in junction-ambient thermal resistances for varying copper areas. The High-K thermal

resistance,

θ

in with two 1-ounce internal power and ground planes and one 2-ounce copper bottom plane for

spreading/sinking heat from the IC component.

Table 2. Thermal Resistance,

θ

Max Dissipation without

Max Dissipation without

Board

Package

θ

JA

Derating (T

Derating (T

High-K

RGW

30.5°C/W

3.27 W

1.8 W

TPS7A4700EVM-094

RGW

21°C/W

4.76 W

2.6 W

4

TPS7A4700EVM-094 Evaluation Module

SLVU741 – May 2012

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