LTC1644 Datasheet, PDF (20/24 Pages) Linear Technology – CompactPCI Bus Hot Swap Controller

LTC1644
APPLICATIO S I FOR ATIO
5V
LONG 5V
PCB EDGE
BACKPLANE BACKPLANE
CONNECTOR CONNECTOR
C9 0.1µF
PER 10
POWER PINS
5V_IN
R22 2.74Ω
C6
Z4
0.01µF
D1
NC
D2
R2
0.007Ω
Q2
IRF7413
R4
10Ω
5V_OUT
C1
R5 _0.047µF
1k
GROUND
17
3V_IN
8
GND
16
3V_SENSE
D1, D2: BAV99
Z4: 1PMT5.0AT3
*ADDITIONAL PINS OMITTED FOR CLARITY
13
5V_IN
14
5V_SENSE
15
GATE
3
5V_OUT
LTC1644*
18
3V_OUT
1644 F13
Figure 13. No 3.3V Supply Application Circuit
BACKPLANE
CONNECTOR
PCB EDGE
BACKPLANE
CONNECTOR
PUSHBUTTON
SWITICH
5V_IN
1.2k
BD_SEL#
GROUND
100Ω
1k ₅
OFF/ON
LTC1644*
8
GND
*ADDITIONAL PINS OMITTED FOR CLARITY
1644 F14
Figure 14. BD_SEL# Pushbutton Toggle Switch
restricts the choice of power MOSFETs to those devices
with very low R_DS(ON). Table 9 lists some power MOSFETs
that can be used with the LTC1644.
Power MOSFETs are classified into two categories: stan-
dard MOSFETs (R_DS(ON) specified at V_GS = 10V) and logic-
level MOSFETs (R_DS(ON) specified at V_GS = 5V). Since
external pass transistors are required for the 3.3V and 5V
supply rails, logic-level power MOSFETs should be used
with the LTC1644.
Overvoltage Transient Protection
Good engineering practice calls for bypassing the supply
rail of any analog 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 large
bypass capacitors slow the rate of rise of the supply
voltage and heavily damp any parasitic resonance of lead
20
or PC track inductance working against the supply bypass
capacitors.
The opposite is true for LTC1644 Hot Swap circuits
mounted on plug-in cards. In most cases, there is no
supply bypass capacitor present on the powered 12V
(12V_IN), –12V (V_EEIN) of the PCB edge connector or on the
3.3V (3V_IN) or the 5V (5V_IN) side of the MOSFET switch. An
abrupt connection, produced by inserting the board into a
backplane connector, results in a fast rising edge applied
on these input supply lines of the LTC1644.
Since there is no bulk capacitance to damp the parasitic
track inductance, supply voltage transients excite para-
sitic resonant circuits formed by the power MOSFET
capacitance and the combined parasitic inductance from
the wiring harness, the backplane and the circuit board
traces. These ringing transients appear as a fast edge on
the input supply lines, exhibiting a peak overshoot up to
2.5 times the steady-state value followed by a damped
1644f