LT1641
APPLICATIO S I FOR ATIO
Hot Circuit Insertion
When circuit boards are inserted into a live backplane, the
supply bypass capacitors on the boards draw high peak
currents from the backplane power bus as they charge up.
The transient currents can permanently damage the con-
nector pins and glitch the system supply, causing other
boards in the system to reset.
The LT1641 is designed to turn on a board’s supply
voltage in a controlled manner, allowing the board to be
safely inserted or removed from a live backplane. The chip
also provides undervoltage and overcurrent protection
while a power good output signal indicates when the
output supply voltage is ready.
Power-Up Sequence
The power supply on a board is controlled by placing an
external N-channel pass transistor (Q1) in the power path
(Figure 5). Resistor R
S
provides current detection and
capacitor C1 provides control of the GATE slew rate.
V
IN
24V
R1
49.9k
1%
R
S
0.025Ω
Q1
IRF530
D1
CMPZ
5248B
C1
R6, 10nF
1k, 5%
R5
10Ω
5%
R3
59k
1%
8
V
CC
1
ON
7
SENSE
6
GATE
FB
2
R4
3.57k
1%
3
R7
24k
5%
LT1641
R2
3.4k
1%
TIMER
5
GND
C2
0.68µF
GND
4
PWRGD
FIgure 5. Typical Application
6
U
Resistor R6 provides current control loop compensation
while R5 prevents high frequency oscillations in Q1.
Resistors R1 and R2 provide undervoltage sensing.
After the power pins first make contact, transistor Q1 is
turned off. If the voltage at the ON pin exceeds the turn-on
threshold voltage, the voltage on the V
CC
pin exceeds the
undervoltage lockout threshold, and the voltage on the
TIMER pin is less than 1.233V, transistor Q1 will be turned
on (Figure 6). The voltage at the GATE pin rises with a slope
equal to 10µA/C1 and the supply inrush current is set at
I
INRUSH
= C
L
• 10µA/C1. If the voltage across the current
sense resistor R
S
gets too high, the inrush current will then
be limited by the internal current limit circuitry which
adjusts the voltage on the GATE pin to maintain a constant
voltage across the sense resistor.
Once the voltage at the output has reached its final value,
as sensed by resistors R3 and R4, the PWRGD pin goes
high.
+
V
OUT
C
L
PWRGD
1641 F05
W
U
U
Figure 6. Power-Up Waveforms
LINER [ LINEAR TECHNOLOGY ]
