7

LT1313

Calculating Junction Temperature

Example: given an output voltage of 12V, an input supply

voltage of 14V, and an output current of 100mA (one VPP

output), and a maximum ambient temperature of 50

°C,

what will the maximum junction temperature be?

Power dissipated by the device will be equal to:

where,

so,

Using Table 1, the thermal resistance will be in the range

of 120

°C/W to 131°C/W depending upon the copper area.

So the junction temperature rise above ambient will be

less than or equal to:

0.275W

× 131°C/W = 36°C

The maximum junction temperature will then be equal to

the junction temperature rise above ambient plus the

maximum ambient temperature or:

For more detailed applications information, see the LT1312

Single PCMCIA VPP Driver/Regulator data sheet.

The fall time from 12V to 0V is set by the output capacitor

and an internal pull-down current source which sinks

about 30mA. This source will fully discharge a 1

µF capaci-

tor in less than 1ms.

Thermal Considerations

Power dissipated by the device is the sum of two compo-

nents: output current multiplied by the input-output differ-

The ground pin current can be found by examining the

Ground Pin Current curves in the Typical Performance

Characteristics section.

Heat sinking, for surface mounted devices, is accom-

plished by using the heat spreading capabilities of the PC

board and its copper traces.

The junction temperature of the LT1313 must be limited to

125

°C to ensure proper operation. Use Table 1, in con-

junction with the typical performance graphs, to calculate

the power dissipation and die temperature for a particular

application and ensure that the die temperature does not

exceed 125

°C under any operating conditions.

Table 1. 16-Pin SO Package*

COPPER AREA

THERMAL RESISTANCE

TOPSIDE

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500 sq mm

2500 sq mm

2500 sq mm

120

°C/W

1000 sq mm

2500 sq mm

2500 sq mm

120

°C/W

225 sq mm

2500 sq mm

2500 sq mm

125

°C/W

1000 sq mm

1000 sq mm

1000 sq mm

131

°C/W

* Device is mounted on topside.