DN253 - LTC1646: Power Supply Isolation Controller Simplifies Hot Swapping the CompactPCI Bus for 5V-/3.3V-Only Applications

Power Supply Isolation Controller Simplifies Hot Swapping the
CompactPCI Bus for 5V-/3.3-Only Applications – Design Note 253
Andy Gardner
and extraction and reports on the state of the supply
voltages via the HEALTHY# signal.
Although ±12V supplies are provided on the CompactPCI
backplanes, many plug-in boards only require 5V and
3.3V. The LTC ®1646 is the ideal power supply isolation
controller for these applications.
LTC1646 Feature Summary
The LTC1646 features can be summarized as follows:
The LTC1646 Hot Swap™ controller is designed to
meet the power supply isolation requirements found
in the CompactPCI hot swap specification PICMG 2.1
for plug-in boards requiring 5V and/or 3.3V supplies.
The chip will turn a board’s supply voltages on and
off in a controlled manner, allowing the card to be
safely inserted or removed without causing glitches
on the power supplies and causing other boards in the
system to reset. It also protects against short circuits,
precharges the bus I/O connector pins during insertion
COMPACT PCI
CIRCUIT CARD
CONNECTOR
(FEMALE)
• Dual level, programmable circuit breakers: this feature is enabled after power-up is complete. If either
supply exceeds current limit for more than 20μs,
the circuit breaker will trip and the chip latches off.
In the event that either supply exceeds three times
the set current limit, all supplies are disabled and
the chip latches off without delay.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and Hot Swap is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
R2
0.007Ω
Q1
1%
1
2 IRF7413
Z2
Z1
5V
R1
0.005Ω
1 1% 2
LONG 5V
3.3V
LONG 3.3V
V(I/O)
R16
1.2k
5%
BD_SEL#
V(I/O)
R14
3k
5%
HEALTHY#
PCI_RST#
R18
10Ω
5%
R19
10Ω
5%
C5
0.1μF
3
R15
3k, 5%
11
5VSENSE
FAULT
4
PWRGD
16
RESETIN
CL (3VOUT)
R5
1k, 5%
R6
2 10k, 5%
RESETOUT
PRECHARGE
13
R13
10Ω, 5%
1
R7
3k
5%
DRIVE
14
R10
C3
18Ω, 5%
4.7nF
R11
10k
5%
C2
0.1μF
3VOUT
LTC1646
GROUND
R9
1k, 5%
Q3
PRECHARGE OUT
MMBT2222A
1V ±20%
IOUT = ±55mA
DATA LINE EXAMPLE
R8
12Ω
5%
3VIN
Figure 1. CompactPCI Application with Only 3.3V and 5V Supplies
3VIN
5VIN
3.3V
5V
RESET#
I/O
DATA BUS
Z1, Z2: BZX84C6V2
03/01/253_conv
C1
0.01μF
5
5VOUT
TIMER
R12
18Ω, 5%
DATA BUS
12
5VIN
OFF/ON
3
3.3V
7.6A
R4
10Ω
5%
9
10
7
3VSENSE GATE 3VOUT
GND
6
I/O PIN 1
CL (5VOUT)
Q2
IRF7413
R3
10Ω
5%
8
3VIN
5V
5A
4
4
C4
0.1μF
R17
1k, 5% 15
3
LOCAL_PCI_RST#
COMPACT PCI
BACKPLANE
CONNECTOR
(MALE)
• Controls 5V and/or 3.3V CompactPCI supplies.
DN253 F01
PCI
BRIDGE
(21154)
• Current limited power-up: the supplies are allowed
to power up in current limit. This allows the chip to
power up boards with widely varying capacitive loads
without tripping the circuit breaker. The maximum
allowable power-up time is programmable using the
TIMER pin and an external capacitor.
• Programmable foldback current limit: a programmable analog current limit with a value that depends
on the output voltage. If the output is shorted to
ground during the power-up cycle, the current limit
drops to keep power dissipation and supply glitches
to a minimum.
• Precharge output: on-chip reference and amplifier
provide 1V for biasing bus I/O connector pins during
card insertion and extraction.
• BD_SEL#, HEALTHY#, PCI_RST# and
LOCAL_PCI_RST# signals are supported.
• Space saving 16-pin SSOP package.
Typical Application
Figure 1 shows a typical application using the LTC1646.
The main 3.3V and 5V inputs to the LTC1646 come from
the medium-length power pins. The long 3.3V and 5V
connector pins are shorted to the medium length 5V
and 3.3V connector pins on the plug-in card and provide
early power for the LTC1646’s precharge circuitry, the
V(I/O) pull-up resistors and the PCI bridge chip. The
BD_SEL# signal is connected to the OFF/ON pin while
the PWRGD pin is connected to the HEALTHY# signal.
The HEALTHY# signal is combined on chip with the
PCI_RST# signal to generate the LOCAL_PCI_RST#
signal which is available at the RESETOUT pin.
The power supplies are controlled by placing external
N-channel pass transistors Q1 and Q2 in the 3.3V and
5V power paths. Resistors R1 and R2 sense overcurrent
conditions, and R5 and C1 provide current control-loop
compensation. Resistors R3 and R4 prevent high frequency oscillations in Q1 and Q2.
Power-Up Sequence
Figure 2 shows a typical power-up sequence.
When the CompactPCI card is inserted, the long 5V,
3.3V and GND connector pins make contact first. The
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LTC1646’s precharge circuit biases the bus I/O pins to
1V during this stage of the insertion. The 5V and 3.3V
medium length pins make contact during the next stage
of the insertion, but the slot power is disabled as long
as the OFF/ON pin is pulled high by the 1.2k pull-up
resistor to V(I/O). During the final stage of the board
insertion, the BD_SEL# short connector pin makes
contact and the OFF/ON pin can be pulled low. This
enables the pass transistor to turn on and a 5μA current
source is connected to the TIMER pin. Each supply is
then allowed to power up at the rate dV/dt = 13μA/
C1 or as determined by the current limit and the load
capacitance, whichever is slower. Current-limit faults are
ignored while the TIMER pin voltage is less than 1.25V.
Once both supplies are within tolerance, the HEALTHY#
signal is pulled low and the LOCAL_PCI_RST# signal
goes high.
GATE
10V/DIV
5VOUT
3VOUT
5V/DIV
TIMER
5V/DIV
BD_SEL#
5V/DIV
HEALTHY#
5V/DIV
LCL_PCI_RST#
5V/DIV
PRECHARGE
5V/DIV
20ms/DIV
DN253 F02
Figure 2. Normal Power-Up Sequence
Conclusion
Using the LTC1646, a CompactPCI board can be made
hot swappable so the system power can remain uninterrupted while the board is being inserted or removed.
With the LTC1646, safe hot swapping becomes as
easy as hooking up an IC, a couple of power FETs and
a handful of resistors and capacitors.
For applications help,
call (408) 432-1900
dn253f_conv LT/TP 0301 345K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 2001
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