LT3014HVIS5 查看數據表（PDF） - Linear Technology

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LT3014HVIS5

20mA, 3V to 80V Low Dropout Micropower Linear Regulator

Linear Technology 

LT3014

APPLICATIONS INFORMATION

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 as long as 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 volt-

age range of 24V to 30V, an output current range of 0mA

to 20mA, and a maximum ambient temperature of 50°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) = 20mA

VIN(MAX) = 30V

IGND at (IOUT = 20mA, VIN = 30V) = 0.55mA

So:

P = 20mA • (30V – 5V) + (0.55mA • 30V) = 0.52W

The thermal resistance for the DFN package 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:

0.52W • 50°C/W = 26°C

The maximum junction temperature will then be equal to

the maximum junction temperature rise above ambient

plus the maximum ambient temperature or:

TJMAX = 50°C + 26°C = 76°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 20mA for 50ms out of every

250ms, what is the junction temperature rise above ambi-

ent? 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)

+ (100μA • 48V) = 0.22W

P2(48V in, 20mA load) = 20mA • (48V – 5V)

+ (0.55mA • 48V) = 0.89W

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

+ (100μA • 72V) = 0.34W

P4(72V in, 20mA load) = 20mA • (72V – 5V)

+ (0.55mA • 72V) = 1.38W

Operation at the different power levels is as follows:

76% operation at P1, 19% for P2, 4% for P3, and

1% for P4.

PEFF = 76%(0.22W) + 19%(0.89W) + 4%(0.34W)

+ 1%(1.38W) = 0.36W

With a thermal resistance in the range of 40°C/W to

62°C/W, this translates to a junction temperature rise

above ambient of 20°C.

3014fd

10

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