LTC1474CS8 datasheet(11/20 Pages) LINER

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LTC1474/LTC1475

APPLICATIONS INFORMATION

LTC1475 Pushbutton On/Off and

Microprocessor Interface

The LTC1475 provides pushbutton control of power on/off

for use with handheld products. A momentary ground on

the ON pin sets an internal S/R latch to run mode while a

momentary ground on the LBI/OFF pin resets the latch to

shutdown mode. See Figure 6 for a comparsion of on/off

operation of the LTC1474 and LTC1475 and Figure 7 for a

comparison of the circuit implementations.

In the LTC1475, the LBI/OFF pin has a dual function as

both the shutdown control pin and the low battery com-

parator input. Since the “OFF” pushbutton is normally

open, it does not affect the normal operation of the low

battery comparator. In the unpressed state, the LBI/OFF

input is the divided down input voltage from the resistive

divider to the internal low battery comparator and will

normally be above or just below the trip threshold of

1.23V. When shutdown mode is desired, the LBI/OFF pin

is pulled below the 0.7V threshold to invoke shutdown.

the depressed switch state is detected by the microcon-

troller through its input. The microcontroller then pulls the

LBI/OFF pin low with the connection to one of its ouputs.

With the LBI/OFF pin low, the LTC1475 powers down

turning the microcontroller off. Note that since the I/O pins

of most microcontrollers have a reversed bias diode

between input and supply, a blocking diode with less than

µA leakage is necessary to prevent the powered down

microcontroller from pulling down on the ON pin.

Figure 19 in the Typical Applications section shows how to

use the low battery comparator to provide a low battery

lockout on the “ON” switch. The LBO output disconnects

the pushbutton from the ON pin when the comparator has

tripped, preventing the LTC1475 from attempting to start

up again until VIN is increased.

RUN

SHUTDOWN

RUN

1474/75 F06

SHUTDOWN

RUN

MODE

ON OVERRIDES LBI/OFF

WHILE ON IS LOW

LTC1474

LTC1475

LBI/OFF

MODE

Figure 6. Comparison of LTC1474 and LTC1475

Run/Shutdown Operation

The ON pin has precedence over the LBI/OFF pin. As seen

in Figure 6, if both pins are grounded simultaneously, run

mode wins.

Figure 18 in the Typical Applications section shows an

example for the use of the LTC1475 to control on/off of a

microcontroller with a single pushbutton. With both the

microcontroller and LTC1475 off, depressing the

pushbutton grounds the LTC1475 ON pin and starts up the

LTC1475 regulator which then powers up the microcon-

troller. When the pushbutton is depressed a second time,

1474/75 F07

RUN

OFF

RUN

LBI/OFF

100k

LTC1474

LTC1475

100k

VIN

Figure 7. Simplified Implementation of

LTC1474 and LTC1475 On/Off

Absolute Maximum Ratings and Latchup Prevention

The absolute maximum ratings specify that SW (Pin 5) can

never exceed VIN (Pin 7) by more than 0.3V. Normally this

situation should never occur. It could, however, if the

output is held up while the supply is pulled down. A

condition where this could potentially occur is when a

battery is supplying power to an LTC1474 or LTC1475

regulator and also to one or more loads in parallel with the

the regulator’s VIN. If the battery is disconnected while the

LTC1474 or LTC1475 regulator is supplying a light load

and one of the parallel circuits is a heavy load, the input

capacitor of the LTC1474 or LTC1475 regulator could be

pulled down faster than the output capacitor, causing the

absolute maximum ratings to be exceeded. The result is

often a latchup which can be destructive if VIN is reapplied.

Battery disconnect is possible as a result of mechanical

stress, bad battery contacts or use of a lithium-ion battery

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