LT1312 데이터 시트보기 (PDF) - Linear Technology

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LT1312

Single PCMCIA VPP Driver/Regulator

Linear Technology 

LT1312

APPLICATIONS INFORMATION

Transient and Switching Performance

The LT1312 is designed to produce minimal overshoot with

capacitors in the range of 1µF 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 40µs, the rise time

for a 10µF 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 1µF 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-

ential voltage IOUT × (VIN – VOUT), and ground pin current

multiplied by supply voltage IGND × VIN.

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.

Table 1. S8 Package*

COPPER AREA

TOPSIDE BACKSIDE

2500 sq mm 2500 sq mm

1000 sq mm 2500 sq mm

225 sq mm 2500 sq mm

1000 sq mm 1000 sq mm

*Device is mounted topside.

THERMAL RESISTANCE

BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq mm

120°C/W

2500 sq mm

120°C/W

2500 sq mm

125°C/W

1000 sq mm

131°C/W

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:

IOUT × (VS – VPPOUT) + (IGND × VIN)

where:

IOUT = 100mA

VIN = 14V

IGND at (IOUT = 100mA, VIN = 14V) = 5mA

so,

PD = 100mA × (14V – 12V) + (5mA × 15V) = 0.275W

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:

TJMAX = 50°C + 36°C = 86°C.

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