DN391 - Pushbutton On/Off Controller Simplifies System Design

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Pushbutton On/Off Controller Simplifies System Design
Design Note 391
Victor Fleury
Introduction
Handheld designers often grapple with ways to de-bounce
and control the on/off pushbutton of portable devices.
Traditional de-bounce designs use discrete logic, flipflops, resistors and capacitors. Other designs include
an onboard microprocessor and discrete comparators
which continuously consume battery power. For high
voltage multicell battery applications, a high voltage
LDO is needed to drive the low voltage devices. All this
extra circuitry not only increases required board space
and design complexity, but also drains the battery when
the handheld device is turned off. Linear Technology addresses this pushbutton interface challenge with a pair
of tiny pushbutton controllers.
The LTC®2950 integrates all the flexible timing circuits
needed to de-bounce the on/off pushbutton of handheld
devices. The part also provides a simple yet powerful
interface that allows for controlled power up and power
down of the handheld device. The LTC2951 offers an
adjustable timer for applications that require more time
during power down. These two micropower, high voltage
(2.7V to 26V) parts are offered in space-saving 8-pin
3mm × 2mm DFN and TSOT-8 packages.
VIN
3V – 26V
VIN
Debounces Turn-On
The circuit in Figure 1 provides manual control of the shutdown pin of a DC/DC converter. To turn on the converter,
the LTC2950 first de-bounces the pushbutton input and
then releases the low leakage enable (EN) output. The turn
on de-bounce time defaults to 32ms and is extendable
by placing an optional capacitor on the ONT pin. This
allows the handheld designer to adjust the length of time
the user must hold down the pushbutton before turning
on power to the device. The timing of Figure 2 illustrates
performance with a noisy ⎯P⎯B pin.
Protect Against Faults at Power Up
The LTC2950 starts a 512ms blanking timer after it enables
the DC/DC converter. If the ⎯K⎯I⎯L⎯L input is not driven high
within this time period, the part automatically shuts off
the converter. This failsafe feature prevents the user from
turning on the handheld device when there is a faulty power
converter or an unresponsive microprocessor.
, LT, LTC and LTM are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
VOUT
DC/DC
BUCK
SHDN
EN
VIN
EN
LTC2950-1
PB
GND
ONT
R1
10k
2V/DIV
INT
INT
KILL
KILL
µP/µC
PB
OFFT
DN391 F01
10ms/DIV
Figure 1. Typical Application with One External Component
06/06/391
DN391 F02
Figure 2. Turn On De-Bounce Timing
Controlled Power Down
To turn off the handheld device, the LTC2950 first
de-bounces the pushbutton input and then asserts the
interrupt output (see Figure 3). The turn off de-bounce
time defaults to 32ms and is extendable by placing an
optional capacitor on the OFFT pin.
The LTC2950 then starts an internal 1024ms blanking
timer that allows the microprocessor to perform its
power down housekeeping functions. At the end of the
timer period, the part shuts down power to the handheld
device by turning off the DC/DC converter. Additionally, the LTC2951 provides an extendable power down
blanking timer (optional KILLT external capacitor) that
accommodates lengthier microprocessor housekeeping
tasks. Note that the LTC2950/LTC2951 de-bounce both
the rising and falling edges of the pushbutton.
Operation Without µP
The LTC2950 is easily adapted for applications that do
not use a µP or µC. Simply connect the ⎯IN⎯T and ⎯K⎯I⎯L⎯L
pins to the output of the DC/DC converter. When the
user presses the pushbutton to turn off system power,
the interrupt output asserts the ⎯K⎯I⎯L⎯L input, which then
shuts off the converter. See Figure 4.
High Voltage, Micropower
The LTC2950 operates from a wide 2.7V to 26.4V input
voltage range to accommodate a wide variety of input
power supplies. This eliminates the need for a high voltage, low power LDO.
The LTC2950 is ideally suited for maximizing the battery
life of a handheld device. When power is turned off to the
handheld device, the LTC2950’s very low quiescent current
(6µA typical) is an insignificant drain on the battery.
Conclusion
The LTC2950 and LTC2951 provide simple, low power,
small footprint solutions to the de-bounce problem. The
LTC2950 integrates adjustable turn on and turn off timing,
plus a fixed 1024ms power down housekeeping timer.
Alternatively, the LTC2951 provides a fixed 128ms turn
on timer, an adjustable turn off timer and an adjustable
power down housekeeping timer. A simple microprocessor
interface protects against faults at power up and allows
for graceful power down.
VIN
9V
VIN
3.3V
VOUT
LT1767-3.3
SHDN
EN
R1
100k
INT
C4
0.1µF
VIN
EN
LTC2950-1
INT
2V/DIV
PB
KILL
GND ONT
OFFT
+
C3*
0.01µF
PB
CONT*
0.033µF
200ms/DIV
DN391 F03
Figure 3. Turn Off De-Bounce Timing
COFFT*
0.033µF
*OPTIONAL
DN391 F04
Figure 4. No µP Application
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2452
www.linear.com
Linear Technology Corporation
dn391f LT 0606 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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