LTC1647 LINER [Linear Technology], LTC1647 Datasheet - Page 28

• Manufacturer Part Number: LTC1647
• Description: Hot Swap Controller
• Manufacturer: LINER [Linear Technology]

LTC4211
To increase the RC delay, the user may either increase
C
actually limit the RC delay due to the reset sink-current
capability of the FAULT pin. Therefore, in order to increase
the RC delay, it is more effective to either increase R
or to put a bleed resistor in parallel with C
For example, increasing R
3.2M decreases the duty cycle to 0.8%.
HOT SWAPPING TWO SUPPLIES
Using two external pass transistors, the LTC4211 can
switch two supply voltages. In some cases, it is necessary
to bring up the dominant supply first during power-up but
ramp them down together during the power-down phase.
The circuit in Figure 20 shows how to program two dif-
ferent delays for the pass transistors. The 5V supply is
powered up first. R1 and C3 are used to set the rise and
fall times on the 5V supply. Next, the 3.3V supply ramps
APPLICATIONS INFORMATION
28
V
AUTO
CC
– V
FILTER
TIMER
RESET
SENSE
GATE
V
or R
V
OUT
CC
AUTO
1
DUTY CYCLE =
. However, increasing C
ON/FAULT
t
t
OFF
OFF
+ t
AUTO
t
2
1
+ t
2
in Figure 18 from 1M to
(t
FILTER
2
<< t
1
, t
AUTO
2
V
AND t
AUTO
FB
t
1
< V
Figure 19. Autoretry Timing
> 2μF will
OFF
REF
to GND.
)
AUTO
3
4
FAST COMPARATOR ARMED
5
LOAD CURRENT
V
up with 20ms delay set by R6 and C2. On the falling edge,
both supplies ramp down together because D1 and D2
bypass R1 and R6.
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 sup-
ply 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 or PC track
inductance working against the supply bypass capacitors.
The opposite is true for LTC4211 Hot Swap circuits mounted
on plug-in cards. In most cases, there is no supply bypass
capacitor present on the powered supply voltage side of
the MOSFET switch. An abrupt connection, produced by
SENSE
REGULATED
6
t
2
= 50mV
GATE
V
OUT
SLOW COMPARATOR ARMED
7
8
t
FILTER
>50mV
V
2μA
REF
B
FPD
10μA
t
RESET
ON/FAULT
t
OFF
4211 F19
4211fb