Datasheet

LTC4218
10
4218ff
APPLICATIONS INFORMATION
The typical LTC4218 application is in a high availability
system that uses a positive voltage supply to distribute
power to individual cards. The basic application circuit
is shown in Figure 1. External component selection is
discussed in detail in the following sections.
Figure 2. Supply Turn-On
Figure 1. 3A, 12V Card Resident Application
Turn-On Sequence
The power supply on a board is controlled by placing
an external N-channel pass transistor (Q1) in the power
path. Note the sense resistor (R
S
) detects current and
the capacitor (C
GATE
) controls gate slew rate. Resistor R1
prevents high frequency oscillations in Q1 and resistor
R
GATE
isolates C
GATE
during fast turn-off.
Several conditions must be present before the external
pass transistor can be turned on. First, the supply V
DD
must exceed its undervoltage lockout level. Next, the
internally generated supply INTV
CC
must cross its 2.65V
undervoltage threshold. This generates a 25μs power-
on-reset pulse which clears the logic’s fault register and
initializes internal latches.
After the power-on-reset pulse, the LTC4218 will go through
the following sequence. First, the UV and OV pins must
indicate that the input power is within the acceptable range.
All of these conditions must be satisfi ed for a duration
of 100ms to ensure that any contact bounce during the
insertion has ended.
The pass transistor is turned on by charging up the GATE
with a 24μA charge pump generated current source
(Figure 2).
The voltage at the GATE pin rises with a slope equal to
24μA/C
GATE
and the supply inrush current is set at:
I
INRUSH
=
C
L
C
GATE
•24μA
When the GATE voltage reaches the MOSFET threshold
voltage, the switch begins to turn on and the SOURCE
voltage follows the GATE voltage as it increases. Once
SOURCE reaches V
DD
, the GATE will ramp up until clamped
by the 6.15V zener between GATE and SOURCE.
As the SOURCE pin voltage rises, so will the FB pin which
is monitoring it. If the voltage across the current sense
resistor (R
S
) gets too high, the inrush current will be limited
by the internal current limiting circuitry. Once the FB pin
crosses its 1.235V threshold and the GATE to SOURCE
voltage exceeds 4.2V, the PG pin will cease to pull low and
indicate that the power is good.
Turn-Off Sequence
The switch can be turned off by a variety of conditions. A
normal turn-off is initiated by the UV pin going below its
1.235V threshold. Additionally, several fault conditions will
turn off the switch. These include an input overvoltage (OV
pin) and overcurrent circuit breaker (SENSE pin). Normally,
the switch is turned off with a 250μA current pulling down
the GATE pin to ground. With the switch turned off, the
SOURCE pin voltage drops which pulls the FB pin below
its threshold. The PG then pulls low to indicate output
power is no longer good.
R6
150k
R7
20k
ADC
R2
226k
C1
0.1μF
R3
20k
12V
12V
4218 F01
R8
10k
R1
10Ω
C
T
0.1μF
C
L
330μF
V
OUT
12V
3A
V
DD
UV
FB
PG
GND
I
MON
R
SET
20k
R
S
2mΩ
Q1
Si7108DN
R
MON
20k
I
SET
C
GATE
0.01μF
R
GATE
1k
GATE SOURCESENSE
–
SENSE
+
LTC4218GN
OV
INTV
CC
TIMER
FLT
+
R4
140k
R5
20k
t1 t2
SLOPE = 24μA/C
GATE
GATE
SOURCE
V
DD
+ 6.15
V
DD
4218 F02