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Power Supply Control the Touch of a Button

Posted: 17 Oct 2005     Print Version  Bookmark and Share



Electronic Engineering Times - Asia 7 September 16 - 30, 2005 www.linear.com52

user to configure longer delays.

Once EN goes high, an internal

512ms blanking timer is started.

This blanking timer is designed to

give sufficient time for the DC/DC

converter to reach its final voltage,

and to allow the 5P enough time

to perform power on tasks. The

KILL pin must be pulled high within

512ms of the EN pin going high.

If not, the DC/DC converter will be

disabled. The next transition on

PB initiates the power off sequence.

Similarly, the default delay is 32ms

and the external OFFT capacitor

allows the user to configure

longer delays. At that point an

interrupt is set, signifying that the

DC/DC converter will be disabled

in 1024ms. Once a system has

finished performing its power down

operations, it can set KILL low (and

thus immediately shut off power),

terminating the internal 1024ms


Figure 1 shows EN controlling

a DC/DC converter for the main

supply to the system. The EN output

is open drain; the LTC2950-1 is

positive logic and the LTC2950-2

is negative logic. The INT and

KILL signals communicate with the


The EN open drain output of

the LTC2950-2 is designed for 10V

so it can switch on/off an external

P-channel power MOSFET. Figure

3 shows the LTC2950-2 (EN)

switching an external MOSFET for

power path control in a two cell Li-

Ion battery application. This allows

a user to connect/disconnect a power

supply or back-up battery to its load

by toggling the PB pin. The INT

Figure 1. Push-Button Control of DC/DC Buck Converter

Power Supply Control Solutions...

at the Touch of a Button

ortable devices need to draw

minimal current when off -

ideally zero - to maximize

battery life. Implementing push-

button on/off control disconnects

everything from the battery except

the push-button control circuitry.

Providing graceful power-up and

power-down with minimal board

space and minimal current draw

is challenging, particularly when

higher voltage battery packs are

used. Linear Technology addresses

the push-button interface challenge

with a pair of tiny controllers.

The LTC.

2950 integrates a

push button controller that provides

enable control for DC/DC converters

with processor interrupt logic and

adjustable debounce timers. The part


High Performance Analog Solutions from Linear Technology

solves the inherent bounce problem

associated with all mechanical

contacts and operates over a 2.7V

to 26V input voltage range to

accommodate a wide variety of input

power supplies. When powering

off, the LTC2950 interrupts the

system processor, alerting it to

perform the necessary power down

and housekeeping tasks. Once

the system completes the power-

down operations, it can command

t h e LT C 2 9 5 0 t o i m m e d i a t e l y

disable power. The LTC2951 offers

additional time for system power

down, configured by an external

capacitor. Offered in tiny 8-pin 2mm

x 3mm DFN and TSOT-8 packages,

the LTC2950 and LTC2951 save

design time, as well as precious

board space for portable instruments

and handheld products.

High Voltage Operation

at Low Current

Operating from 2.7V to 26V, the

high-voltage design provides a

unique advantage as a push-

button de-bounce circuit. It

connects directly to multi-battery

stacks without the need for power-

robbing resistors or level-shifting

circuits. The input pin is rugged,

too, providing 10kV ESD protection.

But unlike most circuits in portable

systems, the push-button interface

must be active continuously - there

is no sleep mode or off state. The

LTC2950 continuously monitors the

push-button input for the next event,

remembering whether the system is

on or off. The supply current is low

while monitoring, minimizing the

drain on the battery.

System Interface

In addition to the debounce function,

there is also a handshake function

that works with the processor or

system logic. There is an interrupt

output and a kill input. During a

"turn-on" event, the LTC2950 will

disable power if the processor does

not come back and release the kill

input within a prescribed time. This

ensures that the power-up occurs

successfully. During the "turn-off"

event, the LTC2950 interrupts the

processor and warns that power will

be turned off. It then waits for the

processor to shut down and return

a kill command before it disables

the power, assuring complete and

successful power-down. If no kill

command comes back, perhaps due

to the processor hanging up, power

is turned off after 512ms.

Referring to the timing diagram

in Figure 2, a transition on PB#

initiates the power on sequence.

The default delay is 32ms and the

external ONT capacitor allows the

LTC2950 LTC2951

Push-Button Control

Push-Button ESD Protection 10kV 10kV

ON debounce delay (Default) 32ms 128ms

Programmable ON time delay

OFF debounce delay (Default) 32ms 32ms

Programmable OFF time delay

System Shutdown delay (Default) 1024ms 128ms

Programmable KILL time delay

Supply Current 65A 65A

Packages 2mm x 3mm DFN 2mm x 3mm DFN


Table 1. LTC2950/LTC2951 Feature Comparison

Figure 2. LTC2950 Timing Sequence

Figure 3. Push-Button Control of P-Channel MOSFET for Power Path Control


Electronic Engineering Times - Asia 7 September 16 - 30, 2005 53

Figure 6. Push-Button Control of a Dual Ideal-Diode for Power Path Control

and KILL pins are connected to the

output of the MOSFET through a

resistive divider. Therefore, KILL will

be pulled low when INT goes low,

killing power immediately. The KILL

pin also serves as a voltage monitor.

When VOUT drops below 6V, the EN

pin is brought high 305s later.

As in the previous circuit, the

circuit in Figure 4 takes advantage

of the precision analog comparator

used for the KILL input so it can

be used as a voltage monitor. It is

driven by a low leakage open drain

output of the 5P. It is also connected

to a resistive divider that monitors

the battery voltage (VIN). When the

battery voltage falls below 5.4V, the

voltage at the KILL pin falls below

0.6V and the EN pin is quickly pulled

low. The DC/DC converter shown

has an internal pull-up current on its

SHDN pin; thus a pull-up resistor on

EN is not needed.

The LTC2951 is identical to the

LTC2950 except that it trades the

adjustable ON timer for an adjustable

KILL timer. Figure 5 shows the same

circuit as in Figure 1 except that the

up sequence. Each output is

monitored and the DONE output

signals that the power-up sequence

is successfully completed. This

releases the KILL input of the

LTC2950. When the push-button

commands the system to power

down, the INT pin pulls the ON

pin of the LTC2924 low to begin

the power-down sequence. The

DONE pin drives KILL low when all

supplies have successfully powered

down and the LTC2950 disconnects

p o w e r u s i n g t h e P - c h a n n e l


Portable and non-portable

devices utilize push-button control

to enable and disable power.

Often this requires a tiny, low

current solution that is capable of

operating from high input voltages.

The LTC2950 and LTC2951 provide

this solution and also interface

with the processor to assure proper

execution of power-up and power-

down routines. In the tiny 2mm x

3mm DFN package with just a few

optional external components, these

devices offer simple and secure

push-button control.

Note: , LTC and LT are registered trademarks of

Linear Technology Corporation. All other trademarks

are the property of their respective owners.

Figure 5. Push-Button Control with Extended Processor Shutdown Time

Figure 7. Push-Button Control of On/Off Power Supply Sequencing

ON debounce time is fixed at 128ms

internally and the external capacitor

connected to the KILLT pin extends

the time allowed for the processor

to complete its shutdown sequence

and release the KILL pin from the

default 128ms to 339ms, as shown

in Figure 5.

Power path control between a

battery and a wall adaptor is shown

in Figure 6. The LTC4413 is a

two-channel ideal diode designed

to reduce heat, voltage drop and

board space as well as preserve

battery life. The device is ideal

for applications requiring an ideal

diode OR function for load sharing

or automatic switchover between

two input power sources. The

LTC4413 has control access to each

ideal diode but in this circuit the EN

output from the LTC2950 is tied to

each of the inputs, activating both

channels at the same time. INT and

KILL are tied together, simplifying

the power-up/down sequence

when a processor handshake is not


Push-button control is not just

for battery-powered systems. Large

systems with multiple supplies

a n d d e m a n d i n g s e q u e n c i n g

requirements can benefit from

simplified on/off control. Figure 7

shows the LTC2950 controlling a

P-channel MOSFET in series with

four supplies that are sequenced

by the LTC2924. When power is

enabled, the ON pin of the LTC2924

is pulled high to begin the power-

Figure 4. Push-Button Control of LDO with Low Battery Monitor

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