12

LT1302/LT1302-5

Table 3. S8 Package, 8-Lead Plastic SO

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500 sq. mm

2500 sq. mm 2500 sq. mm

60

°C/W

1000 sq. mm

2500 sq. mm 2500 sq. mm

62

°C/W

225 sq. mm

2500 sq. mm 2500 sq. mm

65

°C/W

100 sq. mm

2500 sq. mm 2500 sq. mm

69

°C/W

100 sq. mm

1000 sq. mm 2500 sq. mm

73

°C/W

100 sq. mm

225 sq. mm

2500 sq. mm

80

°C/W

100 sq. mm

100 sq. mm

2500 sq. mm

83

°C/W

* Pins 1 and 8 attached to topside copper

N8 Package, 8-Lead DIP:

Thermal Resistance (Junction-to-Ambient) = 100

°C/W

Calculating Temperature Rise

Power dissipation internal to the LT1302 in a boost

regulator configuration is approximately equal to:

The first term in this equation is due to switch “on-

resistance.” The second term is from the switch driver. R

is switch resistance, typically 0.15

forward drop.

The temperature rise can be calculated from:

where:

∆T = Temperature Rise

PI

R

VV

V

IV

R

V

VV

V

IV

R

V

IV

V

V

D

OUT

OUT

D

IN

OUT OUT

IN

OUT

D

IN

OUT OUT

IN

OUT

OUT

D

IN

=

+

−

























−

+

−

























































+

+−

()

2

2

27

separate ground trace up under the package as shown.

The battery and load return should go to the power side of

the ground copper.

Thermal Considerations

The LT1302 contains a thermal shutdown feature which

protects against excessive internal (junction) tempera-

ture. If the junction temperature of the device exceeds the

protection threshold, the device will begin cycling be-

tween normal operation and an off state. The cycling is not

harmful to the part. The thermal cycling occurs at a slow

rate, typically 10ms to several seconds, which depends on

the power dissipation and the thermal time constants of

the package and heat sinking. Raising the ambient tem-

perature until the device begins thermal shutdown gives a

good indication of how much margin there is in the

thermal design.

For surface mount devices heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Experiments have shown that the

heat spreading copper layer does not need to be electri-

cally connected to the tab of the device. The PCB material

can be very effective at transmitting heat between the pad

area attached to pins 1 and 8 of the device, and a ground

or power plane layer either inside or on the opposite side

of the board. Although the actual thermal resistance of the

PCB material is high, the length/area ratio of the thermal

resistance between the layer is small. Copper board stiff-

eners and plated through holes can also be used to spread

the heat generated by the device.

Table 3 lists thermal resistance for the SO package.

Measured values of thermal resistance for several differ-

ent board sizes and copper areas are listed for each

surface mount package. All measurements were taken in

still air on 3/32

be used as a rough guideline in estimating thermal resis-

tance. The thermal resistance for each application will be

affected by thermal interactions with other components as

well as board size and shape.