500

6

CA

Board Area_cm

2 &

°C x cm

2

W

.

6

CA <

125°C - 50°C

3.9 W

- 1.0 °C

W

< 18.23

°C

W

6

CA <

TJ-MAX ± TA-MAX

PIC_LOSS

- 6JC

LMZ23608

SNVS708G – MARCH 2011 – REVISED AUGUST 2015

www.ti.com

Layout Examples (continued)

Figure 59. Bottom View of Evaluation PCB

10.3 Power Dissipation and Thermal Considerations

When calculating module dissipation use the maximum input voltage and the average output current for the

application. Many common operating conditions are provided in the characteristic curves such that less common

applications can be derived through interpolation. In all designs, the junction temperature must be kept below the

rated maximum of 125°C.

For the design case of VIN = 12 V, VOUT = 3.3 V, IOUT = 8 A, and TA-MAX = 50°C, the module must see a thermal

resistance from case to ambient (

θCA) of less than:

(15)

Given the typical thermal resistance from junction to case (

θJC) to be 1.0°C/W. Use the 85°C power dissipation

curves in the Typical Characteristics section to estimate the PIC-LOSS for the application being designed. In this

application it is 3.9 W.

(16)

To reach

θCA = 18.23, the PCB is required to dissipate heat effectively. With no airflow and no external heat-sink,

a good estimate of the required board area covered by 2-oz. copper on both the top and bottom metal layers is:

(17)

As a result, approximately 27.42 square cm of 2-oz. copper on top and bottom layers is the minimum required

area for the example PCB design. This is 5.23 × 5.23 cm (2.06 × 2.06 in) square. The PCB copper heat sink

must be connected to the exposed pad. For best performance, use approximately 100, 8 mil thermal vias spaced

59 mil (1.5 mm) apart connect the top copper to the bottom copper.

26

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