Pushbutton On/Off Controller Includes Optional Automatic Turn-On When Handheld Device is Plugged In

Pushbutton On/Off Controller Includes Optional Automatic
Turn-On When Handheld Device is Plugged In
Vui Min Ho
It is well-known that most mechanical pushbutton
switches bounce when pressed, and that a debounce
circuit is required to produce a clean, usable signal from
the pushbutton. There are many debounce solutions
available—common ones use flip-flops or R-S latches—but
designing and implementing a debounce circuit is not as
trivial as it might seem, especially for handheld devices.
Because a pushbutton debounce circuit
must remain on all the time, a low supply
current is critical for battery-powered
handheld devices. Additionally, the circuit
should be capable of accepting power
from any available standby supply voltage without requiring a linear regulator.
Furthermore, the pushbutton input should
be able to withstand high ESD levels during
operation since it is usually connected
where contact with the human finger is
possible. And finally, the circuit must be
small enough to fit into whatever little
space is left on the printed circuit board.
The LTC2955 pushbutton controller covers
all of these requirements. It generates a
latched enable output from the noisy pushbutton input. The enable output comes in
both active high (LTC2955-1) or active low
(LTC2955-2) options, allowing it to drive
the on/off input of any system or regulator.
The LTC2955 features a voltage monitor pin (ON) that can be used for automatic system turn-on when the device
is plugged into a secondary supply such
as a wall adapter or car battery. This is
a common feature found in handheld
devices where, if you plug in the wall
adapter or charger cable, the device
automatically powers up by itself without a press of the on/off switch.
The LTC2955 is also designed with blanking times after each pushbutton event,
during which all inputs are ignored. This
prevents the EN output from turning on
and off continuously if the pushbutton
is held down or stuck low. These blanking times ensure sufficient time for the
voltage regulator to fully charge up or
discharge its output and allow the system or microprocessor time to perform
power on/off tasks. In addition, the
power-down debounce time is adjustable
using an external capacitor. This allows
the designer to extend the power-down
The LTC2955 is designed to interface
with a microprocessor via the LTC2955’s
INT (interrupt) output pin and KILL input
pin. The LTC2955’s INT output alerts the
microprocessor that the pushbutton is
pressed, allowing the microprocessor to
perform any power-down tasks. Once
these tasks are complete, the microprocessor can communicate—via the
KILL pin—that the system is ready to be
switched off. The user can also force the
Figure 1. Pushbutton on/off control for battery-powered device, with automatic turn-on when the
device is plugged in
LT3060
WALL ADAPTER INPUT
12V
P-CHANNEL
MOSFET
BATTERY INPUT
3.6V
SHDN
LTC4412
VIN
GATE
GND SENSE
VIN
CTL
SEL
STAT
ON
9.6V THRESHOLD
200k
1M
PGD
PB
TMR
LTspice IV
EN
LTC2955-1
2.21M
circuits.linear.com/559
10 | July 2012 : LT Journal of Analog Innovation
system to power-down if the microprocessor fails to respond to the interrupt
signal (KILL pin remains high). This is
the familiar user-holds-the-button-down
for a duration greater than the defined
power-off period. The power-off period
is adjustable through the capacitor at the
TMR pin and it can be made as long as
required to prevent accidental turn-off.
INT
GND
KILL
µP
design features
The LTC2955 pushbutton controller generates a latched enable
output from the noisy pushbutton input. The enable output
comes in both active high or active low options, allowing
it to drive the on/off input of any system or regulator.
time in cases where some systems need
more time to perform power-down tasks.
MOSFET to reduce the voltage drop across
TOP OFto
BOARD
the diode connected
the 3.6V supply.
The LTC2955 minimizes components by
operating directly from inputs as low as
a 1.5V single-cell battery up to a 36V multicell stack—with no additional boost
regulator or LDO required. The low quiescent current of 1.2μA extends the battery
life. The device is available in a spacesaving 10-lead 3mm × 2mm DFN package and 8-lead ThinSOT™ package.
The LTC2955-1 ON pin monitors the
12V input through the resistive divider R1
and R2. When the user plugs in the wall
adapter, the 12V supply becomes present.
The LTC2955-1 detects that the ON pin is
high and pulls the EN (enable) pin high
after a 32ms debounce time and turns on
the voltage regulator, applying power to
the system. This allows automatic system
turn-on when the user plugs in the wall
adapter. The system can also be turned on
by pressing the pushbutton. The LTC2955
alerts the microprocessor that the 12V supply is present or absent by pulling the
PGD output pin high or low, respectively.
HANDHELD WITH AUTOMATIC
TURN-ON WHEN PLUGGED IN
Figure 1 shows a typical LTC2955-1
application for a handheld device. The
3.6V supply is produced directly from the
handheld’s battery; the 12V secondary
supply comes from a wall adapter. Both
the 3.6V and 12V inputs are connected
to the LT3060 regulator input via diodeOR so that either supply can power the
system. The LTC4412 is an ideal diode
controller that controls the P-channel
PUSHBUTTON PIN ESD PROTECTION
The LTC2955 PB (pushbutton) input
is protected to ESD levels of up to
±25kV HBM with respect to ground. This
protection level exists during all modes
Figure 2. LTC2955-1 waveforms
TURN ON PULSE
SHORT PULSE
VERSATILE PUSHBUTTON INPUT
The LTC2955 requires only a few external components for most applications,
as shown in Figure 1. Except for the
logic-level pins used to interface with
the microprocessor, most of the pins can
withstand a maximum voltage of 36V,
precluding the need for external supplies or resistor dividers. Designs remain
flexible in the face of high input supply voltages, especially when no typical
board-level supply (e.g., 5V) is available.
The PB input is designed to operate in
harsh and noisy environments. The pin
Figure 3. Pushbutton input
LONG PULSE
LTC2955
EXTERNAL
SUPPLY
PB
EN
of operation including power-down,
BOTTOM
BOARD is dispower-up or when
theOFsupply
connected from the battery. When the
pushbutton pin is hit with an ESD strike
during operation, the part remains in
its current logic state. The device does
not reset or latch up and there is no
need to cycle the supply to recover.
TURNS ON
STAYS ON
TURNS OFF
VLDO
OPTIONAL
PULLUP
RESISTOR
D1
900k
PB
0.8V
–
+
INT
INTERRUPT
INTERRUPT
July 2012 : LT Journal of Analog Innovation | 11
The PB input is designed to operate in harsh and noisy environments.
The pin can withstand both positive and negative voltages up to ±36V.
This allows for long cable runs between the pushbutton switch and the
LTC2955, where even if the input rings, it will not cause damage to the part.
VIN
LTC2955-1
VLDO
LTC2955-2
2µA
D2
EN
VOUT
OPTIONAL
PULLUP
RESISTOR
VIN
M1
EXTERNAL
SUPPLY
OPTIONAL VIN
VOUT
PULLUP
VOLTAGE
RESISTOR
REGULATOR
EN
CONTROL
900k
EN
SHDN
EN
CONTROL
LTC2955-2
Figure 4. LTC2955-1 EN output
can withstand both positive and negative
voltages up to ±36V. This allows for long
cable runs between the pushbutton switch
and the LTC2955, where even if the input
rings, it will not cause damage to the part.
Figure 3 shows the pushbutton pin connection and internal circuitry. The internal
900k pull-up resistor allows connection
of the pin directly to the pushbutton
switch (with the other terminal grounded)
without requiring an external pull-up
resistor. If an external pull-up resistor is
desired in applications where the pushbutton switch is leaky, this optional pull-up
resistor can be tied to any voltage up to
the 36V maximum as shown. The internal
diode D1 blocks the external supply current from flowing into the device, preventing unnecessary current consumption.
12 | July 2012 : LT Journal of Analog Innovation
Figure 5. LTC2955-2 EN output
VERSATILE ENABLE OUTPUT
Figure 4 shows the LTC2955-1 active
high EN pin driving the SHUTDOWN input
of a voltage regulator. The LTC2955-1
EN pin pulls high to 4.3V with an internal
2µ A pull-up current in active mode. If a
higher VOH voltage is required, an optional
external pull-up resistor can be added
to pull this pin above 4.3V as shown.
The diode D2 blocks the external supply current from flowing into the device.
The EN pin can be pulled high up to 36V.
Figure 5 shows the LTC2955-2 active low
EN pin driving a P-channel MOSFET to
control the system supply. The LTC2955-2
EN pin pulls high through an internal 900k
resistor during the inactive mode. In the
active mode with the pin low, this 900k
resistor is disconnected from the supply to
minimize the quiescent current consumed
by the 900k resistor. If a VOH lower than
the supply voltage is required, this pin
can be tied to the external supply through
an optional pull-up resistor as shown.
The ON input and SEL inputs can withstand
voltages up to 36V. This makes it easy to
connect these pins directly to the high voltage supply without requiring a resistive
divider, and thus minimize the quiescent
current consumed by the resistive divider.
CONCLUSION
The LTC2955 is a family of micropower
(1.2µ A), wide input voltage range (1.5V to
36V) pushbutton controllers. These parts
lower system cost and preserve battery life by integrating a rugged pushbutton input, a flexible enable output
and a simple microprocessor interface
that provides intelligent power-up and
power-down. The device is available
in space-saving 10-lead 3mm × 2mm
DFN and 8-lead ThinSOT packages. n