LTC1422C Просмотр технического описания (PDF) - Linear Technology
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LTC1422C Datasheet PDF : 16 Pages
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LTC1422
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.
The LTC1422 is designed to turn a board’s supply voltage
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 system reset signal to indicate when board
supply voltage drops below a programmable voltage.
Power Supply Ramping
The onboard power supply is controlled by placing an
external N-channel pass transistor in the power path
(Figure 1). R1 provides current fault detection and R2
prevents high frequency oscillation. By ramping up the
gate of the pass transistor 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.
VCC + 10V
SLOPE = 10µA/C1
GATE
VCC
VOUT
t1
t2
1422 F02
Figure 2. Supply Turn-On
equal to 10µA/C1 (Figure 2), where C1 is the external
capacitor connected between the GATE pin and GND.
The ramp time for the supply is equal to: t = (VCC • C1)/
10µA. After the ON pin has been pulled low for more than
40µs, the GATE is immediately pulled to GND.
Voltage Monitor
The LTC1422 uses a 1.232V bandgap reference, precision
voltage comparator and a resistive divider to monitor the
output supply voltage (Figure 3).
R1
Q1
VCC
R2
10Ω
8
76
VCC SENSE GATE
2
5
ON
FB
LTC1422
1
RESET
TIMER GND
3
4
C2
+
R3
VOUT
CLOAD
C1
R4
1422F01
Figure 1. Supply Control Circuitry
R1
Q1
VCC
R2
+ VOUT
CLOAD
LTC1422
8
7
6
VCC SENSE GATE
2
ON
LOGIC
COMP 2
C1
R3
FB 5
R4
TIMER
1.232V
REFERENCE
12µA
1
Q2
µP
RESET
When power is first applied to the chip, the gate of the
N-channel (Pin 6) is pulled low. After the ON pin is held
high for at least one timing cycle, the charge pump is
turned on. The voltage at GATE begins to rise with a slope
3
C2
4
1422 F03
Figure 3. Supply Monitor Block Diagram
7
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