Datasheet

7
LT1641-1/LT1641-2
164112fc
ON
1641-1 F02
GATE
5V
t
PLHON
1.313V
1V
t
PHLON
1.233V
FB
1641-1 F03
PWRGD
1V
t
PLHFB
1.313V
1V
t
PHLFB
1.233V
V
CC
– SENSE
1641-1 F04
GATE
V
CC
t
PHLSENSE
47mV
TI I G DIAGRA S
WUW
Figure 2. ON to GATE Timing
Figure 3. FB to PWRGD Timing
Figure 4. SENSE to GATE Timing
APPLICATIO S I FOR ATIO
WUU
U
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 chip 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.
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.