LTC4240_15 Datasheet, PDF (16/28 Pages) Linear Technology – CompactPCI Hot Swap Controller with I2C Compatible Interface

LTC4240
APPLICATIO S I FOR ATIO
stage in the hot plug sequence, indicating that the
LTC4240 is in reset mode with all power switches off
(BD_SEL# is still pulled high to long 5V).
The 12V and –12V supplies make contact at this stage.
Zener clamps Z1 and Z2 plus shunt RC snubbers R13-
C4 and R14-C5 help protect the 12V_IN and V_EEIN pins,
respectively, from large transient voltages during hot
insertion and short-circuit conditions.
The signal pins also connect at this point. This includes
the HEALTHY# signal connecting to the PWRGD pin
and the PCI_RST# signal connecting to the RESETIN
pin. The PWRGD and RESETIN signals are combined
internally with Bit 3 (C3) of the I²C command latch (see
Send Byte protocol) to generate the LOCAL_PCI_RST#
signal, which is available at the RESETOUT pin.
4. Short pins make contact. BD_SEL# signal connects to
the OFF/ON pin. This starts the electrical part of the
connection process. If the BD_SEL# signal is grounded
on the backplane, then the electrical connection pro-
cess starts immediately. Note that the electrical con-
nection process can be interrupted with the Send Byte
protocol of the I²C serial interface.
System backplanes that do not ground the BD_SEL#
signal will instead have circuitry that detects when
BD_SEL# has made contact with the plug-in board. The
backplane logic can then control the power up process
by pulling BD_SEL# low. Figure 4 illustrates the power
up sequence. The mating of BD_SEL# is represented by
the high to low transition of the BD_SEL# signal.
Power-Up Sequence
Two external N-channel power MOSFETs isolate the 3.3V
and 5V power paths, while two internal MOS switches
isolate the 12V and –12V power paths. (See front page
Application Circuit). Sense resistors R1 and R2 provide
current limit and fault detection for the 3V_IN and 5V_IN
supplies, while R5 and C1 provide current control loop
compensation. Current fault detection for the 12V and
–12V supplies is done internally.
A high to low transition on BD_SEL# causes the voltages
on the TIMER, GATE, 3V_OUT, 5V_OUT, 12V_OUT and V_EEOUT
pins to begin ramping (see Figure 4). The TIMER pin
capacitance is charged by an 11.5µA current source while
the GATE capacitance is charged by a 65µA current source.
Concurrently, an internal charge pump turns on the gates
of the internal power switches that isolate the 12V and
–12V supplies. All faults are ignored during the time that
the voltage at the TIMER pin remains below 5.5V. In order
to avoid faults due to the charging of the bulk output
capacitors, all output voltages must settle before the
TIMER pin reaches 5.5V. See TIMER section for more
details.
The 5V_OUT and 3V_OUT supply outputs will ramp up accord-
ing to the slowest of the following slew rates:
dV ₌ 65µA _{, or =} ^I_LIMIT(5V)^{– I}_{LOAD(5V) ,}
(1a)
dt C1
C _{LOAD(5 VOUT)}
or = ^{ILIMIT(3V)– ILOAD(3V)}
(1b)
C _{LOAD(3 VOUT)}
TIMER
10V/DIV
GATE
10V/DIV
12V_OUT
10V/DIV
5V_OUT
10V/DIV
3V_OUT
10V/DIV
V_EEOUT
10V/DIV
BD_SEL#
5V/DIV
LCL_PCI_RST#
5V/DIV
HEALTHY#
5V/DIV
10ms/DIV
Figure 4. Normal Power-Up Sequence
4240 F04
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