AN813

Vishay Siliconix

www.vishay.com

2

Document Number: 71236

12-Dec-03

THERMAL PERFORMANCE

Junction-to-Foot Thermal Resistance

(the Package Performance)

Thermal performance for the 3-pin SC-70 measured as

junction-to-foot thermal resistance is 285_C/W typical,

340_C/W maximum. Junction-to-foot thermal resistance for

the 6-pin SC70-6 is 105_C/W typical, 130_C/W maximum —

a nearly two-thirds reduction compared with the 3-pin device.

The “foot” is the drain lead of the device as it connects with the

body. This improved performance is obtained by the increase

in drain leads from one to four on the 6-pin SC-70. Note that

these numbers are somewhat higher than other LITTLE FOOT

devices due to the limited thermal performance of the Alloy 42

lead-frame compared with a standard copper lead-frame.

Junction-to-Ambient Thermal Resistance

(dependent on PCB size)

state, compared with 180_C/W for the 6-pin SC-70. Maximum

ratings are 430_C/W for the 3-pin device versus 220_C/W for

the 6-pin device. All figures are based on the 1-inch square

FR4 test board.The following table shows how the thermal

resistance impacts power dissipation for the two different

pin-outs at two different ambient temperatures.

SC-70 (3-PIN)

Room Ambient 25 _C

Elevated Ambient 60 _C

SC-70 (6-PIN)

Room Ambient 25 _C

Elevated Ambient 60 _C

NOTE: Although they are intended for low-power applications,

devices in the 6-pin SC-70 will handle power dissipation in

excess of 0.5 W.

Testing

To aid comparison further, Figures 3 and 4 illustrate

single-channel SC-70 thermal performance on two different

board sizes and two different pad patterns. The results display

the thermal performance out to steady state and produce a

graphic account of the thermal performance variation between

for the single 3-pin and 6-pin SC-70 are as follows:

LITTLE FOOT SC-70

3-Pin

6-Pin

1) Minimum recommended pad pattern

(see Figure 4) on the EVB.

410.31_C/W

329.7_C/W

2) Industry standard 1” square PCB with

maximum copper both sides.

360_C/W

211.8_C/W

The results show that designers can reduce thermal

device rather than the 3-pin device. In this example, a 80_C/W

reduction was achieved without an increase in board area. If

increasing board size is an option, a further 118_C/W reduction

could be obtained by utilizing a 1-inch square PCB area.

Time (Secs)

FIGURE 3.

Comparison of SC70-3 and SC70-6 on EVB

0

1

400

80

160

100

1000

240

10

0.5 in x 0.6 in EVB

3-pin

320

Time (Secs)

FIGURE 4.

Comparison of SC70-3 and SC70-6 on 1”

Square FR4 PCB

0

1

400

80

160

100

1000

240

10

1” Square FR4 PCB

320

6-pin

3-pin

6-pin