ltc1647-2 Linear Technology Corporation, ltc1647-2 Datasheet - Page 12

• Manufacturer Part Number: ltc1647-2
• Description: Dual Hot Swap Controllers
• Manufacturer: Linear Technology Corporation

LTC1647-1/LTC1647-2/LTC1647-3
APPLICATIO S I FOR ATIO
pulls the ON pin up. C3 delays GATE turn-on until the
voltage at the ON pin exceeds V
GATE ramps up at 10 A/C1 until Q1 conducts. If V
still shorted to GND, the cycle repeats. The ramp interval
is about t
voltage of the external MOSFET.
Hot Circuit Insertion
When circuit boards are inserted into a live backplane or
a device bay, the supply bypass capacitors on the board
can draw huge transient currents from the backplane or
the device bay power bus as they charge up. The transient
currents can damage the connector pins and glitch the
system supply, causing other boards in the system to
reset or malfunction.
The LTC1647 is designed to turn two positive supplies on
and off in a controlled manner, allowing boards to be safely
inserted or removed from a live backplane or device bay.
The LTC1647 can be located before or after the connector
as shown in Figure 8. A staggered PCB connector can
sequence pin conections when plugging and unplugging
circuit boards. Alternatively, the control signal can be
generated by processor control.
Ringing
Good engineering practice calls for bypassing the supply
rail of any circuit. Bypass capacitors are often placed at the
supply connection of every active device, in addition to one
or more large value bulk bypass capacitors per supply rail.
If power is connected abruptly, the bypass capacitors slow
the rate of rise of voltage and heavily damp any parasitic
resonance of lead or trace inductance working against the
supply bypass capacitors.
The opposite is true for LTC1647 Hot Swap circuits on a
daughterboard. In most cases, on the powered side of the
MOSFET switch (V
present. An abrupt connection, produced by plugging a
board into a backplane connector, results in a fast rising
edge applied to the V
12
t
DELAY
RAMP
= –R3•C3•ln[1–(V
= V
TH
CC
U
•C1/10 A where V
) there is no supply bypass capacitor
CC
line of the LTC1647.
U
IH
– V
IH
OL
. The delay time is
W
)/(V
TH
ON
is the threshold
– V
OL
U
)]
OUT
is
No bulk capacitance is present to slow the rate of rise and
heavily damp the parasitic resonance. Instead, the fast
edge shock excites a resonant circuit formed by a combi-
nation of wiring harness, backplane and circuit board
parasitic inductances and MOSFET capacitance. In theory,
the peak voltage should rise to 2X the input supply, but in
practice the peak can reach 2.5X, owing to the effects of
voltage dependent MOSFET capacitance.
The absolute maximum V
17V; any circuit with an input of more than 6.8V should be
scrutinized for ringing. A well-bypassed backplane should
not escape suspicion: circuit board trace inductances of as
little as 10nH can produce sufficient ringing to overvoltage
V
Check ringing with a fast storage oscilloscope (such as a
LECROY 9314AL DSO) by attaching coax or a probe to V
and GND, then repeatedly inserting the circuit board into
the backplane. Figures 9a and 9b show typical results in a
12V application with different V
amplitude reaches 22V, breaking down the ESD protection
diode in the process.
There are two methods for eliminating ringing: clipping
and snubbing. A transient voltage suppressor is an effec-
tive means of limiting peak voltage to a safe level.
Figure 10 shows the effect of adding an ON Semiconduc-
tor, 1SMA12CAT3, on the waveform of Figure 9.
Figures 11a and 11b show the effects of snubbing with
different RC networks. The capacitor value is chosen as
10X to 100X the MOSFET C
selected for best damping—1 to 50 depending on the
value of parasitic inductance.
Supply Glitching
LTC1647 Hot Swap circuits on the backplane are generally
used to provide power-up/down sequence at insertion/
removal as well as overload/short-circuit protection. If a
short-circuit occurs at supply ramp-up, the circuit breaker
trips. The partially enhanced MOSFET, Q1, is easily dis-
connected without any supply glitch.
CC
.
CC
potential for the LTC1647 is
OSS
CC
lead lengths. The peak
under bias and R is
CC