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LT3011

3011f

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 52°C/W to

64°C/W depending on the copper area. So, the junction

temperature rise above ambient will be approximately

equal to:

1.28W • 58°C/W = 74°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 + 74°C = 124°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 follow:

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

+ (200μA • 48V) = 0.23W

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

+ (1mA • 48V) = 2.20W

P3 (72VIN, 5mA load) = 5mA (72V – 5V)

+ (200μA • 72V) = 0.35W

P1 (72VIN, 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 52°C/W to 64°C/W,

this translates to a junction temperature rise above ambi-

ent of 33°C to 41°C.

High Temperature Operation

Care must be taken when designing LT3011 applications to

operate at high ambient temperatures. The LT3011 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 LT3011. 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 the 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.

Leakage in capacitors, or from solder ﬂux left after insuf-

ﬁcient board cleaning, adversely affects the low quies-

cent current operation. Consider junction temperature

increase due to power dissipation in both the junction

and nearby components to ensure maximum speciﬁca-

tions are not violated for the LT3011E/LT3011H/LT3011I

or external components.

Protection Features

The LT3011 incorporates several protection features which

make it ideal for use in battery-powered circuits. In ad-

dition to the normal protection features associated with

monolithic regulators, such as current limiting and thermal

limiting, the device is protected against reverse-input

voltages, and reverse voltages from output-to-input.

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