DN93 - PCMCIA Socket Voltage Switching

PCMCIA Socket Voltage Switching – Design Note 93
Why Your Portable System Needs SafeSlot™ Protection
Doug La Porte
Introduction
Most portable systems have built-in PCMCIA sockets
as the sole means of expansion. The requirements of
the PCMCIA specification have led to some confusion
among system designers. This Design Note will attempt
to lessen the confusion and highlight other practical
system issues.
3.3V switch is critical to assure that the specified 3.0V
minimum is maintained. The VPP supply must source
12V at up to 120mA and 3.3V or 5V at lesser currents.
The VPP supply is intended solely for flash memory
programming. The 120mA current requirement allows
writing to flash devices and simultaneously erasing two
other parts as required by many flash drives.
Host power delivery to the PC card socket flows through
two paths: the main VCC supply pins and the VPP programming pins. Both supplies are switchable to different
voltages to accommodate a wide range of card types. The
VCC main card supply must be capable of delivering up
to 1A at either 3.3V or 5V. The 1A rating is an absolute
maximum derived from the contact rating of 500mA per
pin for both VCC pins and assumes that both pins are
in good condition and current is shared equally. One of
the most stringent actual current requirements is during
hard drive spin-up. Present hard drives require 5V at
600mA to 800mA for a short duration during spin-up.
Current draw drops to 300mA to 420mA during read
and write operations. A low switch resistance on the
The host PCMCIA socket designer also has several
other practical aspects of the design to consider. The
exposed socket pins are vulnerable to being shorted
by foreign objects such as paper clips. In addition the
users will attempt to install damaged cards. In short,
once in the hands of the consumer, the designer and
manufacturer have little control over use and abuse.
To ensure a robust system and a satisfied customer,
switch protection features such as current limiting and
thermal shutdown are a necessity. The nature of the
PC cards and portable systems requires the card being
powered on and off as needed to conserve power. Many
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and SafeSlot is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
D1
MBRS130LT3
L1
22μH
C1
47μF
+
+
SW
VCC
C2
33μF
SELECT
3.3V OR 5V
SENSE
LT1301
ILIM
SHDN
PGND
N/C
GND
5V
L1 = SUMIDA CD75-220k
C1 = AVX TPSD476M016R0150
C2 = AVX TPSD336M020R0200
3.3V
5VIN
VDD
SHDN
3VIN
SENSE
VPP1
VPPOUT
LTC1472
PCMCIA
CONTROLLER
(CIRRUS LOGIC, VADEM, INTEL)
VPPENO
VCC(IN)
VPPEN1
VCC(OUT)
VCCENO
0.1
10k
VPP2 PC CARD
SOCKET
VCC
+
1μF
VCCEN1
GND
Figure 1. Typical LTC1472 Application with the LT1301 3.3V Boost Regulator
12/94/93_conv
PC cards have large input capacitance and draw over
2W. The power up/down sequencing can put demanding
transient requirements on your system power supply.
To make the transient response of the system supply
manageable, the PCMCIA switch should have breakbefore-make switching, controlled rise and fall times
and current limiting. The slowed rise time coupled with
current limiting are critical in controlling the immense
in-rush current difficulties seen when charging the large
input capacitance of many cards
LTC®1472: Complete VCC and VPP PCMCIA Switch
Matrix with SafeSlot Protection
The LTC1472 is a complete, fully integrated VCC and VPP
switch matrix that addresses all of the PCMCIA socket
switching needs. Figure 1 shows a typical LTC1472 application used in conjunction with the LT®1301 to supply
12V for flash memory programming. The LTC1472’s
logic inputs allow direct interfacing with both logic high
and logic low industry standard controllers without any
external glue logic. The LTC1472 is available in the space
saving narrow 16-pin SOIC package. The VCC switch’s
RDS(ON) is 0.14Ω to support the 1A current requirement.
The VCC output is switched between 3.3V, 5V and high
impedance. The VPP output pin is switched between
0V, VCC, 12V and high impedance. Table 1 shows the
VCC and VPP truth tables.
Table 1. LTC1472 Truth Table
VCC Switch
VCCEN0
0
1
0
0
VPP Switch
VPPEN0
0
0
1
1
VCCEN1
0
0
1
1
VCC(OUT)
off
5V
3.3V
off
VPPEN1
0
1
0
1
VPP0UT
0V
VCC(IN)
VPPIN
Hi-Z
The LTC1472 features SafeSlot protection. The built-in
SafeSlot current limiting and thermal shutdown features
are vital to ensuring a robust and reliable system. The
VCC current limit is above 1A to maintain compatibility
with all existing cards yet provide protection. The
VPP current limit is above 120mA to also maintain
compatibility. All switches are break-before-make type
Data Sheet Download
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Linear Technology Corporation
with controlled, slowed rise and fall times for minimal
system supply impact. In-rush current, from even the
largest card input capacitance, is kept under control
by the LTC1472’s slowed rise-time switching and current limiting.
The LTC1472 has on-chip charge pumps for switch
driving. For this reason, the device does not require a
continuous 12V source. Most of the time the LT1301
is in shutdown mode, consuming only 10μA. The
LT1301 becomes operational only during flash memory
programming. The LTC1472 itself conserves power by
going to a low 1μA standby mode when VCC and VPP
outputs are switched off. The use of the LT1301 is
optional. Any suitable 12V supply can be directly connected to the VPPIN pin. Caution should be exercised
when using a general purpose 12V supply; make certain
that it does not have spikes or transients exceeding the
flash memory 14V maximum voltage rating and that
the regulation is within the 5% flash memory tolerance.
Conclusion
PCMCIA sockets are the preferred method of expansion
in portable systems. As these devices proliferate to less
sophisticated users, there will be greater opportunity
for abuse. To counter this trend the portable system
design must take safeguards to protect the system. The
high level of integration, SafeSlot protection features
and controlled rise and fall switching make the LTC1472
the ideal solution for portable systems.
Linear Technology has a family of PCMCIA socket voltage
control products to suit a broad range of customer’s
needs. Table 2 lists the present part offerings. For
assistance on your specific design needs, call Linear
Technology.
Table 2. Linear Technology’s PCMCIA Host Socket
Voltage Control Products
Part Number
Remarks
LT1312
VPP Linear Regulator
LT1313
Dual Slot VPP Linear Regulator
LT1314
Low Cost VCC and VPP Switch Matrix
LT1315
Dual Low Cost VCC and VPP Switch Matrix
LTC1470
Complete SafeSlot Protected VCC Switch Matrix
LTC1471
Dual Complete SafeSlot Protected VCC Switch
Matrix
LTC1472
Complete SafeSlot Protected VCC and Switch
Matrix
For applications help,
call (408) 432-1900
dn93f_conv LT/GP 1294 160K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1994
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