9

LT3014B

3014bf

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

+ (0.55mA • 48V) = 0.89W

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

+ (100

µA • 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.

+ 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.

Protection Features

The LT3014B incorporates several protection features

which 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 ther-

mal limiting, the device is protected against reverse-input

voltages, and reverse voltages from output to input.

Current limit protection and thermal overload protection

are intended to protect the device against current overload

conditions at the output of the device. For normal opera-

tion, the junction temperature should not exceed 125

°C.

The input of the device will withstand reverse voltages of

80V. Current flow into the device will be limited to less than

6mA (typically less than 100

µA) and no negative voltage

will appear at the output. The device will protect both itself

and the load. This provides protection against batteries

which can be plugged in backward.

The ADJ pin can be pulled above or below ground by as

much as 7V without damaging the device. If the input is left

open circuit or grounded, the ADJ pin will act like an open

circuit when pulled below ground, and like a large resistor

(typically 100k) in series with a diode when pulled above

ground. If the input is powered by a voltage source, pulling

the ADJ pin below the reference voltage will cause the

device to current limit. This will cause the output to go to

an unregulated high voltage. Pulling the ADJ pin above the

reference voltage will turn off all output current.

In situations where the ADJ pin is connected to a resistor

divider that would pull the ADJ pin above its 7V clamp

voltage if the output is pulled high, the ADJ pin input

current must be limited to less than 5mA. For example, a

resistor divider is used to provide a regulated 1.5V output

from the 1.22V reference when the output is forced to 60V.

The top resistor of the resistor divider must be chosen to

limit the current into the ADJ pin to less than 5mA when the

ADJ pin is at 7V. The 53V difference between the OUT and

ADJ pins divided by the 5mA maximum current into the

ADJ pin yields a minimum top resistor value of 10.6k.

In circuits where a backup battery is required, several

different input/output conditions can occur. The output

voltage may be held up while the input is either pulled to

ground, pulled to some intermediate voltage, or is left open

circuit. Current flow back into the output will follow the

curve shown in Figure 4. The rise in reverse output current

above 7V occurs from the breakdown of the 7V clamp on

the ADJ pin. With a resistor divider on the regulator output,

this current will be reduced depending on the size of the

resistor divider.

When the IN pin of the LT3014B is forced below the OUT

pin or the OUT pin is pulled above the IN pin, input current

will typically drop to less than 2

µA. This can happen if the

input of the LT3014B is connected to a discharged (low

voltage) battery and the output is held up by either a

backup battery or a second regulator circuit.

APPLICATIO S I FOR ATIO

OUTPUT VOLTAGE (V)

0

50

45

40

30

35

25

20

15

10

5

0

8

3014 F04

2

135

79

46

10

CURRENT FLOWS

INTO OUTPUT PIN

ADJ PIN

ESD CLAMP

Figure 4. Reverse Output Current