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LT3029

3029f

APPLICATIONS INFORMATION

Calculating Junction Temperature

Example: Channel 1’s output voltage is set to 1.8V. Chan-

nel 2’s output voltage is set to 1.5V. Each channel’s input

voltage is 2.5V. Each channel’s output current range is

0mA to 500mA. The application has a maximum ambient

temperature of 50°C. What is the LT3029’s maximum

junction temperature?

The power dissipated by each channel equals:

IOUT(MAX)(VIN – VOUT) + IGND(VIN)

where for each output:

IOUT(MAX) = 500mA

VIN = 2.5V

IGND at (IOUT = 500mA, VIN = 2.5V) = 8.5mA

So, for output 1:

P = 500mA (2.5V – 1.8V) + 8.5mA (2.5V) = 0.37W

For output 2:

P = 500mA (2.5V – 1.5V) + 8.5mA (2.5V) = 0.52W

The thermal resistance is in the range of 35°C/W to 40°C/W,

depending on the copper area. So, the junction temperature

rise above ambient temperature approximately equals:

(0.37W + 0.52W) 39°C/W = 34.7°C

The maximum junction temperature then equals the maxi-

mum ambient temperature plus the maximum junction

temperature rise above ambient temperature, or:

TJMAX = 50°C + 34.7°C = 84.7°C

Protection Features

The LT3029 regulator incorporates several protection fea-

tures that make it ideal for use in battery-powered circuits.

In addition to the normal protection features associated

with monolithic regulators, such as current limiting and

thermal limiting, the device protects itself against reverse

input voltages and reverse voltages from output to input.

The two regulators have independent inputs, a common

GND pin and are thermally coupled. However, the two

channels of the LT3029 operate independently. Each

channel’s output can be shut down independently, and

a fault condition on one output does not affect the other

output electrically, unless the thermal shutdown circuitry

is activated.

Current limit protection and thermal overload protection

protect the device against current overload conditions at

each output of the LT3029. For normal operation, do not

allow the junction temperature to exceed 125°C (LT3029E/

LT3029I/LT3029MP) or 150°C (LT3029H). The typical ther-

mal shutdown temperature threshold is 165°C and the

circuitry incorporates approximately 5°C of hysteresis.

Each channel’s input withstands reverse voltages of 22V.

Current ﬂow into the device is limited to less than 1mA

(typically less than 100μA) and no negative voltage appears

at the respective channel’s output. The device protects

both itself and the load against batteries that are plugged

in backwards.

The LT3029 incurs no damage if either channel’s output

is pulled below ground. If the input is left open-circuit,

or grounded, the output can be pulled below ground by

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