LTC1421-2.5_15 Datasheet, PDF (8/24 Pages) Linear Technology – Hot Swap Controller

LTC1421/LTC1421-2.5
APPLICATIONS INFORMATION
Hot Circuit Insertion
When circuit boards are inserted into a live backplane, the
supply bypass capacitors on the board can draw huge
transient currents from the backplane power bus as they
charge up. The transient currents can cause permanent
damage to the connector pins and cause glitches on the
system supply, causing other boards in the system to
reset. At the same time, the system data bus can be
disrupted when the board’s data pins make or break
connection.
The LTC1421 is designed to turn a board’s supply voltages
on and off in a controlled manner, allowing the board to be
safely inserted or removed from a live backplane. The chip
also provides a disable signal for the board’s data bus
buffer during insertion or removal and provides all the
necessary supply supervisory functions for the board.
Power Supply Ramping
The power supplies on a board are controlled by placing
external N-channel pass transistors in the power path
(Figure 3). R1 and R2 provide current fault detection. By
ramping the gate of the pass transistor up at a controlled
rate, the transient surge current (I = C • dV/dt) drawn from
the main backplane supply can be limited to a safe value
when the board makes connection.
12V
1 ^R1 2
5V
3
4
23
22
Q1
21
1 ^R2 2
Q2
3
4
20 19
18 17
+
V_OUTHI
C_LOAD
R_RAMP +
16
V_OUTLO
C_LOAD
V_CCLO SETLO GATELO V_OUTLO V_CCHI
1
CON1
2
CON2
LTC1421
SETHI GATEHI V_OUTHI
10
RAMP
C_RAMP
1421 F03
Figure 3: Supply Control Circuitry
When power is first applied to the chip, the gates of both
N-channels, GATELO and GATEHI are pulled low. After the
connection sense pins, CON1 and CON2 are both held low
for at least 20ms, a 20µA reference current is connected
from the RAMP pin to GND. The voltage at GATEHI begins
to rise with a slope equal to 20µA/C_RAMP (Figure 4), where
C_RAMP is an external capacitor connected between the
12V
V_OUTHI
SLOPE = 20µA/C_RAMP
5V
V_OUTLO
t₁
t₂
1421 F4a
Figure 4. Supplies Turning On
RAMP and GATEHI pins. The voltage at the GATEHI pin is
clamped one Schottky diode drop below GATELO.
The ramp time for each supply is equal to: t = (V_CC)
(C_RAMP)/20µA. During power down the gates are actively
pulled down by two internal NFETs.
A negative supply voltage can be controlled using the
CPON pin as shown in Figure 5.
When the board makes connection, the transistor Q3 is
turned off because it’s gate is pulled low to –12V by R4.
CPON is also pulled to –12V. When the charge pump is
turned on, CPON is pulled to V_CCLO and the gate of Q3 will
ramp up with a time constant determined by R4, R5 and
C2. When the charge pump is turned off, CPON goes into
a high impedance state, the gate of Q3 is discharged to V_EE
with a time constant determined by R4 and C2, and Q3
turns off.
–12V FROM
CONNECTOR
5V
CPON
– 12V
0V
B
– 12V
0V
V_EE
– 12V
~1ms
C2
0.047µF
Q3
1/2 MMDF3N0HD
R4
20k
+
5%
B
R5
16k
5%
9
CPON
V_EE
–12V
C_LOAD ^1A
LTC1421
~1ms
1421 F05
Figure 5. Negative Supply Control
8