DN346 - PCI Express Power and Mini Card Solutions

PCI Express Power and Mini Card Solutions – Design Note 346
Mitchell Lee and Vladimir Ostrerov
Introduction
PCI Express is the third generation of PCI (Peripheral
Component Interconnect) technology used to connect
I/O perhipheral devices in computer systems. It is
intended as a general purpose I/O device interconnect
that meets the needs of a wide variety of computing
platforms such as desktop, mobile, server and communications. It also specifies the electrical and mechanical
attributes of the backplane, connectors and removable
cards in these systems.
PCI Express Mini Card also provides two power sources:
3.3V and 1.5V. The auxiliary voltage (3.3VAUX) is sourced
over the same pins as the primary 3.3V supply and is
available during the standby/suspend state to support
wake-event processing on the Mini Card. Table 2 summarizes PCI Express Mini Card power specifications.
Power Requirements
Each add-in card connector requires two power rails:
12V and 3.3V, along with a third, optional 3.3V auxiliary rail. If the designer elects to provide the auxiliary
3.3V rail in his system or if the platform supports the
WAKE# signal, the 3.3VAUX rail must be supplied to all
connectors.
Table 2. Summary of PCI Express Mini Card Supply
Requirements
In PCI Express, as in PCI, the Hot Swap™ controller is
resident on the platform side of the system, delivering
power to the connectors only after a card is inserted.
This way, empty and otherwise unused connectors
are safely unpowered, reducing the risk of inadvertent
power supply faults, connector damage and simplifying
power-up when a card is inserted into the connector.
Table 1 shows the PCI Express power supply rail
specifications per connector, based on the number of
connectors in the system.
Table 1. Summary of PCI Express Power Supply
Requirements
POWER RAIL
×1
×4/×8
CONNECTOR CONNECTOR
×16
CONNECTOR
12V
Supply Current
Capacitive Load
0.5A
300μF
2.1A
1000μF
4.4A (Up to 5.5A)
2000μF
3.3V
Supply Current
Capacitive Load
3A
1000μF
3A
1000μF
3A
1000μF
3.3VAUX
Supply Current
Capacitive Load
0.375A
150μF
0.375A
150μF
0.375A
150μF
09/04/346_conv
There is no requirement for supply sequencing in either
PCI Express or PCI Express Mini Card; the supplies may
come up or go down in any order.
POWER RAIL
PRIMARY POWER
AUXILIARY POWER
3.3V
Peak Supply Current
1A
—
1.5V
Peak Supply Current
0.5A
—
3.3VAUX
Peak Supply Current
0.33A
0.25A
Note: All values are maximums.
Circuit Solutions
Figure 1 shows a circuit for a ×16 connector PCI Express
system, implemented with an LTC®1647 dual Hot Swap
controller. As shown, port power is gated by a control
signal applied through 4.7k resistors to the ON pins.
The FAULT outputs are cross-coupled with the ON pins
such that if an overcurrent fault occurs on either supply, the unaffected rail also shuts down. Shutdown is
reset by toggling the control signal into the off state
for at least 100μs.
The supply rails slew at less than 1.5V/ms, well under
any of the PCI Express limits. Thus the only alteration
necessary to scale the design for ×1, ×4 and ×8 connector systems is to replace the 12V supply sense resistor
with 70mΩ (×1) or 15mΩ (×4/×8) as appropriate.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and Hot Swap and ThinSOT are trademarks of Linear Technology Corporation. All
other trademarks are the property of their respective owners.
7mΩ
500mW IRF7413
VIN
12V
100nF
10μF
12mΩ
500mW IRF7413
10Ω
VIN
3.3V
100nF
1
OFF/RESET ON
5
2
4.7k
4
3
VCC1
15 13
16
S1 G1
14 12
100nF
6
VCC
(OPTIONAL
LOGIC
OFF ON
CONTROL
INPUT)
10nF
3
AUX OUTPUT
3.3V
375mA
5
S
4
10nF
G
LTC4210-1CS6
ON
GND
TIMER
S2 G2
VCC2
10μF
100Ω
100Ω
10Ω
1k
10nF
100mΩ Si1400DL
VIN
3.3V
VOUT
3.3V
3A
10μF
1k
4.7k
VOUT
12V
5.5A
2
1
220nF
FAULT2
ON1
DN346 F02
LTC1647-3CGN
ON2
FAULT1
Figure 2. Optional 3.3VAUX Port Power with SelfReseting, Current-Limited Circuit Breaker Operates
Autonomously or Under Logic Control
GND
DN346 F01
8
Figure 1. PCI Express Port Power for ×16 Connector
Satisfies dV/dt Specifications, Soft-Starts Full Load
Capacitance and Guards Against Short Circuits and
Overloads
Figure 2 shows a current limited circuit breaker for the
optional 3.3VAUX rail, using the LTC4210 Hot Swap
controller in a 6-lead ThinSOT™ package. With the
ON pin (3) hard-wired to 3.3V, the LTC4210 operates
autonomously, featuring a timed current limit to protect
against overloads. Should the circuit breaker trip in the
presence of a sustained fault, the LTC4210 automatically
restarts after a time delay, requiring no intervention on
the part of the PCI Express port manager.
PCI Express Mini Card
Mini Card circuit requirements differ from PCI Express
in several ways, but most importantly the Hot Swap
controller resides on the card itself. In this configuration,
it is important for the Hot Swap controller to monitor
the incoming power and switch on only when those
voltages are fully established.
Figure 3 shows a Mini Card Hot Swap control circuit
implemented with an LTC1645, with undervoltage and
overcurrent monitoring on both the 3.3V and 1.5V supplies. A fault on either supply rail causes both channels
to shut down simultaneously. An open-drain signal
at the ON pin permits optional logic control over the
LTC1645. Similar to the LTC1647, pulsing ON low resets
the LTC1645 after a fault.
For 3.3VAUX, simply add the circuit of Figure 2 in parallel
at the 3.3V output. Note that with both circuits enabled
and providing power to the 3.3V output, the available
current is greater than 1.5A.
A complete definition of the PCI Express power delivery
requirements is found in the PCI Express Card Electromechanical Specification and PCI Express Mini Card
Electromechanical Specification. Further information
is available at www.pcisig.com.
VOUT
3.3V
CLOAD 1A
40mΩ Si1400DL
VIN
3.3V
10Ω 82mΩ Si1400DL
VIN
1.5V
CLOAD
10V
VOUT
1.5V
0.5A
10Ω
13 12
14
4990Ω
10
1820Ω
9
124Ω
8
1370Ω
VCC1
1
2
S1 G1 VCC2
FB
ON
COMPOUT
COMP
3
S2 G2
FAULT
LTC1645CS
+
GND
TIMER
7
11
RESET
10k
100nF
6
4
124Ω
1370Ω
5
DN346 F03
RESET
330nF
Figure 3. PCI Express Mini Card Hot Swap Circuit Resides on Mini Card
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900
dn346f_conv LT/TP 0904 435K • PRINTED IN THE USA
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