10

LT1312

Table 1. S8 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 topside.

Calculating Junction Temperature

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

voltage of 14V, an output current of 100mA, 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:

Transient and Switching Performance

The LT1312 is designed to produce minimal overshoot with

capacitors in the range of 1

µF to 10µF. Larger capacitor

values can be used with a slowing of rise and fall times.

The positive output slew rate is determined by the 330mA

current limit and the output capacitor. The rise time for a

0V to 12V transition is approximately 40

µs, the rise time

for a 10

µF capacitor is roughly 400µs (see the Transient

Response curves in the Typical Performance Characteris-

tics section).

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 LT1312 must be limited to

125

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

tion 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.