LINEAR TECHNOLOGY LTC2950 Technical data
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Pushbutton On/Off Controller Simplifi es System Design
Design Note 391
Victor Fleury
Introduction
Handheld designers of ten grapple with ways to de -bounce
and control the on/off pushbutton of portable devices.
Traditional de-bounce designs use discrete logic, fl ipfl ops, 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 fl exible 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.
Debounces Turn-On
The circuit in F igure 1 provides manual control o f the shutdown pin of a DC/DC converter. To turn on the converter,
the LTC2950 fi rst de-bounces the pushbutton input and
then releases the low leakage enable (EN) ou tput. 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 adjus t the leng th 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 star ts a 512ms blank ing 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 w hen 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.
V
IN
3V – 26V
V
IN
PB
GND
LTC2950-1
ONT OFFT
INT
KILL
EN
V
IN
SHDN
R1
10k
DC/DC
BUCK
V
OUT
INT
µP/µC
KILL
DN391 F01
Figure 1. Typical Application with One External Component
06/06/391
EN
2V/DIV
PB
10ms/DIV
Figure 2. Turn On De-Bounce Timing
DN391 F02
Controlled Power Down
To turn off the handheld device, the LTC2950 fi rst
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 ver y low quiescent current
(6µA typical) is an insignifi cant drain on the battery.
Conclusion
The LTC2950 and LTC2951 provide simple, low power,
small footprint solutions to the de-bounce problem. The
L T C 2 9 5 0 i n t e g r a t e s a d j u s t a b l e t u r n o n a n d t u r n o f f t i m i n g ,
plus a fi xed 1024ms power down housekeeping timer.
Alternatively, the LTC2951 provides a fi xed 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.
EN
INT
2V/DIV
PB
200ms/DIV
Figure 3. Turn Off De-Bounce Timing
DN391 F03
0.1µF
V
IN
9V
V
C4
IN
LTC2950-1
PB
GND ONT OFFT
C
*
ONT
0.033µF
INT
KILL
EN
C
OFFT
0.033µF
*
*OPTIONAL
V
IN
LT1767-3.3
SHDN
V
OUT
3.3V
R1
100k
+
C3*
0.01µF
DN391 F04
Figure 4. No µP Application
Data Sheet Download
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
For applications help,
call (408) 432-1900, Ext. 2452
dn391f LT 0606 305K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2006
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