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LT3012EFE

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

Linear Technology 

LT3012

APPLICATIONS INFORMATION

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 50mA, 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) = 50mA

VIN(MAX) = 30V

IGND at (IOUT = 50mA, VIN = 30V) = 1mA

So:

P = 50mA • (30V – 5V) + (1mA • 30V) = 1.28W

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:

1.31W • 50°C/W = 65.5°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 + 65.5°C = 115.5°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 50mA 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)

+ (200μA • 48V) = 0.23W

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

+ (1mA • 48V) = 2.20W

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

+ (200μA • 72V) = 0.35W

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

+ (1mA • 72V) = 3.42W

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%(0.23W) + 19%(2.20W) + 4%(0.35W)

+ 1%(3.42W) = 0.64W

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 26°C to 38°C.

High Temperature Operation

Care must be taken when designing LT3012 applications to

operate at high ambient temperatures. The LT3012 works

at elevated temperatures but erratic operation can occur

due to unforeseen variations in external components. Some

tantalum capacitors are available for high temperature

operation, but ESR is often several ohms; capacitor ESR

above 3Ω is unsuitable for use with the LT3012. Ceramic

capacitor manufacturers (Murata, AVX, TDK, and Vishay

Vitramon at this writing) now offer ceramic capacitors that

are rated to 150°C using an X8R dielectric. Device instability

will occur if output capacitor value and ESR are outside

design limits at elevated temperature and operating DC

voltage bias (see information on capacitor characteristics

under Output Capacitance and Transient Response). Check

each passive component for absolute value and voltage

ratings over the operating temperature range.

Leakages in capacitors or from solder flux left after

insuficient board cleaning adversely affects low

quiescent current operation. The output voltage resistor

divider should use a maximum bottom resistor value of

124k to compensate for high temperature leakage, setting

divider current to 10μA. Consider junction temperature

increase due to power dissipation in both the junction and

nearby components to ensure maximum specifications are

not violated for the device or external components.

3012fd

11

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