LT3013(RevC) Просмотр технического описания (PDF) - Linear Technology

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LT3013
(Rev.:RevC)

250mA, 4V to 80V Low Dropout Micropower Linear Regulator with PWRGD

Linear Technology 

LT3013

APPLICATIONS INFORMATION

For surface mount devices, heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Copper board stiffeners and plated

through-holes can also be used to spread the heat gener-

ated by power devices.

The following tables list thermal resistance for several

different board sizes and copper areas. All measurements

were taken in still air on 3/32” FR-4 board with one ounce

copper.

Table 1. TSSOP Measured Thermal Resistance

COPPER AREA

TOPSIDE

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

2500 sq mm

2500 sq mm

40°C/W

1000 sq mm

2500 sq mm

45°C/W

225 sq mm

2500 sq mm

50°C/W

100 sq mm

2500 sq mm

62°C/W

Table 2. DFN Measured Thermal Resistance

COPPER AREA

TOPSIDE

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

2500 sq mm

2500 sq mm

40°C/W

1000 sq mm

2500 sq mm

45°C/W

225 sq mm

2500 sq mm

50°C/W

100 sq mm

2500 sq mm

62°C/W

The thermal resistance junction-to-case (θJC), measured

at the exposed pad on the back of the die, is 16°C/W.

Continuous operation at large input/output voltage dif-

ferentials and maximum load current is not practical

due to thermal limitations. Transient operation at high

input/output differentials is possible. The approximate

thermal time constant for a 2500sq mm 3/32” FR-4 board

with maximum topside and backside area for one ounce

copper is 3 seconds. This time constant will increase as

more thermal mass is added (i.e., vias, larger board, and

other components).

For an application with transient high power peaks, average

power dissipation can be used for junction temperature

calculations if the pulse period is significantly less than

the thermal time constant of the device and board.

Calculating Junction Temperature

Example 1: Given an output voltage of 5V, an input voltage

range of 8V to 12V, an output current range of 0mA to

250mA, and a maximum ambient temperature of 30°C,

what will the maximum junction temperature be?

The power dissipated by the device will be equal to:

IOUT(MAX) • (VIN(MAX) – VOUT) + (IGND • VIN(MAX))

where:

IOUT(MAX) = 250mA

VIN(MAX) = 12V

IGND at (IOUT = 250mA, VIN = 12V) = 8mA

So:

P = 250mA • (12V – 5V) + (8mA • 12V) = 1.85W

The thermal resistance will be in the range of 40°C/W to

62°C/W depending on the copper area. So the junction

temperature rise above ambient will be approximately

equal to:

1.85W • 50°C/W = 92.3°C

The maximum junction temperature will then be equal to

the maximum junction temperature rise above ambient

plus the maximum ambient temperature or:

TJMAX = 30°C + 92.3°C = 122.3°C

Example 2: Given an output voltage of 5V, an input voltage

of 48V that rises to 72V for 5ms(max) out of every 100ms,

and a 5mA load that steps to 200mA for 50ms out of ev-

ery 250ms, what is the junction temperature rise above

ambient? Using a 500ms period (well under the time

constant of the board), power dissipation is as follows:

P1(48V in, 5mA load) = 5mA • (48V – 5V)

+ (200μA • 48V) = 0.23W

P2(48V in, 50mA load) = 200mA • (48V – 5V)

+ (8mA • 48V) = 8.98W

P3(72V in, 5mA load) = 5mA • (72V – 5V)

+ (200μA • 72V) = 0.35W

P4(72V in, 50mA load) = 200mA • (72V – 5V)

+ (8mA • 72V) = 13.98W

3013fc

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