Datasheet

LTC4211
17
4211fb
OPERATION
SOFT-START WITH CURRENT LIMITING
During the second timing cycle, the inrush current was
described by Equation 4. Note that there is a one-to-one
correspondence in the inrush current to C
LOAD
. If the inrush
current is large enough to cause a voltage drop greater
than 50mV across the sense resistor, an internal servo
loop controls the operation of the 10μA current source at
the GATE pin to regulate the load current to:
I
LIMIT(SOFTSTART)
=
50mV
R
SENSE
(6)
For example, the inrush current is limited to 5A when
R
SENSE
= 0.01Ω.
In this fashion, the inrush current is controlled and C
LOAD
is charged up slowly during the soft-start cycle.
The timing diagram in Figure 7 illustrates the operation
of the LTC4211 in a normal power-up sequence with lim-
ited inrush current as described by Equation 6. At Time
Point5, the GATE pin voltage begins to ramp and the power
MOSFET starts to charge C
LOAD
. At Time Point 5A, the inrush
current causes a 50mV voltage drop across R
SENSE
and
the internal servo loop engages, limiting the inrush cur-
rent to a fixed level. At Time Point 6, the GATE pin voltage
continues to ramp as C
LOAD
charges until V
OUT
reaches its
final value. The charging current reduces, and the internal
servo loop disengages. At the end of the soft-start cycle
(Time Point 7), RESET is high and SLOW COMP is armed.
Figure 7. Normal Power-Up Sequence (with Current Limiting in Second Timing Cycle)
PLUG-IN CYCLE
FIRST TIMING CYCLE
TIMER
ON
V
CC
12 345 6 7 8 9
RESET PULLED LOW DUE TO POWER BAD
10
200μA
4211 F07
2μA2μA
V
REF
10μA
GATE
V
OUT
POWER BAD
V
FB
< V
REF
5A
SLOW COMPARATOR ARMEDCHECK FOR GATE < 0.2V
FAST COMPARATOR ARMED
CHECK FOR FILTER LOW (<V
REF
– 80mV)
CHECK FOR FAULT HIGH (>V
REF
+ 50mV)
ON GOES LOW
GATE
V
OUT
I
LOAD
RESET
SOFT-START CYCLE
SECOND TIMING CYCLE
POWER GOOD
V
FB
> V
REF
LOAD CURRENT IS
REGULATING AT 50mV/R
SENSE