LT3011I Просмотр технического описания (PDF) - Linear Technology

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LT3011I

50mA, 3V to 80V Low Dropout Micropower Linear Regulator with PWRGD

Linear Technology 

LT3011

APPLICATIONS INFORMATION

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

(LT3011E/LT3011I) or 150°C (LT3011H).

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 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 backwards.

The ADJ pin of the adjustable device 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 try and force the current

limit out of the output. 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 volt-

age 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

1.24V 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

pin is 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 5. 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 regula-

tor output, this current will be reduced depending on the

size of the resistor divider.

When the IN pin of the LT3011 is forced below the OUT

pin or the OUT pin is pulled above the IN pin, input cur-

rent will typically drop to less than 2μA. This can happen

if the input of the LT3011 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 pin will have no effect on the reverse output

current when the output is pulled above the input.

160

140

120

ADJ PIN CLAMP

(SEE ABOVE)

100

80

60

40

TJ = 25°C

VIN = 0V

CURRENT FLOWS

20

INTO OUTPUT PIN

VOUT = VADJ

0

0 1 2 3 4 5 6 7 8 9 10

OUTPUT VOLTAGE (V)

3011 F05

Figure 5. Reverse Output Current

3011f

13

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