LTC2952CF datasheet(18/30 Pages) LINER

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LTC2952

2952fa

APPLICATIONS INFORMATION

The timing diagram in Figure 8 shows the PB pin being

debounced and setting an internal ON/OFF signal. Note that

a high at the internal ON/OFF signal indicates that the last

event was a turn-on command and a low at the internal

ON/OFF signal indicates that the last event was a turn-off

command. Here specifically the turn-on command is a

result of a pushbutton on event and the turn-off command

is a result of a pushbutton off event.

Note that a complete pushbutton consists of a push event

and a release event. The push event (falling edge) on and

off debounce durations on the PB pin can be increased

beyond the fixed internal 26ms by placing a capacitor on

the ONT and OFFT pins respectively. The following equa-

tions describe the additional debounce time that a push

event at the PB pin must satisfy before it is recognized as

a valid pushbutton on or off.

tONT = CONT • (9.3MΩ)

tOFFT = COFFT • (9.3MΩ)

CONT and COFFT are the ONT and OFFT external program-

ming capacitors respectively.

Note that during the push event of the pushbutton off, the

INT pin is asserted low after the initial 26ms debounce

duration. The INT pin asserts low when the PB pin is

held low during the OFFT debounce duration and during

the shutdown sequence. If the PB pin pulls high before

the OFFT time ends, the INT pin immediately turns high

impedance. On the other hand, if the PB pin is still held

low at the end of the OFFT time, the INT pin continues to

assert low throughout the ensuing shutdown sequence.

On a release event (rising edge) of the pushbutton switch

following a valid push event, the PB pin must be continu-

ously held above its rising threshold (0.8V) for a fixed

26ms internal debounce time.

In a typical application, the PB pin is connected to a

pushbutton switch. If the switch exhibits high leakage

current (>10μA), connecting an external pull-up resistor

to V1, V2 and/or VS (depending on the application) is

recommended. Furthermore, if the pushbutton switch is

physically located far from the LTC2952’s PB pin, signals

may couple onto the high impedance PB input. Placing

a 0.1μF capacitor from the PB pin to ground reduces the

impact of signal coupling. Additionally, parasitic series

inductance may cause undesirable ringing at the PB pin.

This can be minimized by placing a 5k resistor in series

and located next to the switch.

Figure 8. Pushbutton Debounce Timing Diagram

2952 F08

ONT CAP

OFFT CAP

INTERNAL

ON/OFF SIGNAL

tOFFT

tONT

<tONT

tONT

tDB,OFF

26ms

<tOFFT

tDB,OFF

26ms

tDB,ON

26ms

tDB,ON

26ms

tDB,OFF

26ms

<tDB,ON

26ms

<tDB,OFF

26ms

tDB

26ms

tDB

26ms

tDB

26ms

<tDB

26ms

INVALID PB

PUSH EVENT

INVALID PB

‘TURN-OFF’ EVENT

VALID PB

‘TURN-ON’ EVENT

INVALID PB

PUSH EVENT

INVALID PB

RELEASE EVENT

INT

LTC2952CF Datasheet(PDF) 18 Page - Linear Technology

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