LTC2954CTS8-1-PBF データシートの表示（PDF） - Linear Technology

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LTC2954CTS8-1-PBF

Pushbutton On/Off Controller with µP Interrupt

Linear Technology 

LTC2954

APPLICATIONS INFORMATION

low 512ms after it went high. Note that the LTC2954 does

not sample KILL and PB until after the 512ms internal timer

has expired. The reason PB is ignored is to ensure that

the system is not forced off while powering on. Once the

512ms timer expires (t4), the release of the PB pin is then

debounced with an internal 32ms timer. The system has

now properly powered on and the LTC2954 monitors PB

and KILL for a turn-off command while consuming only

6μA of supply current.

A high to low transition on PB (t5) starts the power-off

sequence debounce timer. In order to assert the interrupt

output (INT), PB must stay low continuously (PB high

resets debounce timer) for 32ms (t6–t5). At the comple-

tion of the power-down debounce timer (t6), an internal

interrupt timer keeps the interrupt output low for at least

32ms, even if PB is released between t6 and t7. If PB is

low at the end of this 32ms internal timer (t7), the external

adjustable power-down timer is started. The capacitor

placed at the PDT pin will determine the time period of

this timer. If the pushbutton is released prior to 16 cycles

of the PDT pin, the interrupt output will go high (t8). Note

that the enable output is not directly changed by this

interrupt pulse. The function of the interrupt signal is to

initiate a software shutdown. At t9, the μP has performed

its power-down functions and asserted the KILL input

low. This releases the enable output, which in turn shuts

down system power. Note that if the pushbutton is held

long enough to count 16 cycles at the PDT pin, the enable

pin would be released immediately after the 16th cycle.

The system is now in its reset state where the LTC2954 is

in low power mode (6μA) and PB is monitored for a high

to low transition.

Aborted Power-On Sequence

The power-on sequence is aborted when the KILL remains

low at the end of the 512ms blanking time. Figure 2 is a

simplified version of an aborted power-on sequence. At

time tABORT, since KILL is still low, EN pulls low (thus

turning off the DC/DC converter).

μP Turns Off Power During Normal Operation

Once the system has powered on and is operating nor-

mally, the μP can turn off power by setting KILL low, as

shown in (Figure 3). At time tKILL, KILL is set low by the

μP. This immediately pulls EN low, thus turning off the

DC/DC converter.

DC/DC Turn Off Blanking

When the DC/DC converter is turned off, it can take a sig-

nificant amount of time for its output to decay to ground. It

is desirable to wait until the output of the DC/DC converter

is near ground before allowing the user (via PB) to restart

the converter. This condition guarantees that the μP has

always powered down completely before it is restarted.

Figure 4 shows the μP turning power off. After a low on

KILL releases enable, the internal 256ms timer ignores the

PB pin. This is shown as tEN/EN, LOCKOUT in (Figure 4).

PB

POWER ON

tEN/EN, LOCKOUT

PB IGNORED

tKILL

DC/DC

TURNS OFF

PB

PB BLANKING

256ms

DC/DC

TURNS OFF

(INTERNAL

SIGNAL)

EN

EN

μP SETS

KILL LOW

KILL

XXX DON’T CARE

2954 F03

Figure 3. μP Turns Off Power (LTC2954-1)

KILL

XXX DON’T CARE

μP SETS

KILL LOW

2954 F04

Figure 4. DC/DC Turn-Off Blanking (LTC2954-1)

2954fb

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