LT3027 データシートの表示（PDF） - Linear Technology

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LT3027

Dual 100mA, Low Dropout, Low Noise, Micropower Regulator with Independent Inputs

Linear Technology 

LT3027

APPLICATIONS INFORMATION

The power dissipated by each channel of the device will be

equal to:

IOUT(MAX)(VIN(MAX) – VOUT) + IGND(VIN(MAX))

where (for the first channel):

IOUT(MAX) = 100mA

VIN(MAX) = 6V

IGND at (IOUT = 100mA, VIN = 6V) = 2mA

so:

P1 = 100mA(6V – 3.3V) + 2mA(6V) = 0.28W

and (for the second channel):

IOUT(MAX) = 50mA

VIN(MAX) = 6V

IGND at (IOUT = 50mA, VIN = 6V) = 1mA

so:

P2 = 50mA(6V – 2.5V) + 1mA(6V) = 0.18W

The thermal resistance will be in the range of 40°C/W to

60°C/W depending on the copper area. So the junction

temperature rise above ambient will be approximately

equal to:

(0.28W + 018W)(60°C/W) = 27.8°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 + 27.8°C = 77.8°C

Protection Features

The LT3027 regulator incorporates several protection fea-

tures which makes it ideal for use in battery-powered cir-

cuits. In addition to the normal protection features asso-

ciated with monolithic regulators, such as current limiting

and thermal limiting, the devices are 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

12

20V. Current flow into the device will be limited to less than

1mA (typically less than 100µA) 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 output of the LT3027 can be pulled below ground

without damaging the device. If the input is left open circuit

or grounded, the output can be pulled below ground by 20V.

The output will act like an open circuit; no current will flow

out of the pin. If the input is powered by a voltage source,

the output will source the short-circuit current of the de-

vice and will protect itself by thermal limiting. In this case,

grounding the SHDN pins will turn off the device and stop

the output from sourcing the short-circuit current.

The ADJ pins 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 pins 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.

In situations where the ADJ pins are connected to a resis-

tor divider that would pull the pins above their 7V clamp

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

rent must be limited to less than 5mA. For example, a re-

sistor divider is used to provide a regulated 1.5V output from

the 1.22V reference when the output is forced to 20V. 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 13V difference between output and ADJ

pin divided by the 5mA maximum current into the ADJ pin

yields a minimum top resistor value of 2.6k.

In circuits where a backup battery is required, several dif-

ferent input/output conditions can occur. The output volt-

age may be held up while the input is either pulled to ground,

pulled to some intermediate voltage or is left open circuit.

When the IN pins of the LT3027 are forced below the cor-

responding OUT pins or the OUT pins are pulled above the

IN pins, input current will typically drop to less than 2µA.

This can happen if the input of the device is connected to

a discharged (low voltage) battery and the output is held

up by either a backup battery or a second regulator circuit.

The state of the SHDN pins will have no effect on the reverse

output current when the output is pulled above the input.

3027f

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