Datasheet

LTC4211
15
4211fb
OPERATION
The C
TIMER
value is vital to ensure a proper start-up and
reliable operation. A system may not start up if a timing
period is set too short relative to the time needed for the
output voltage to ramp up from zero to its rated value.
Conversely, this timing period should not be too long as
an output short can occur at start-up causing the exter-
nal MOSFET to overheat. A good starting point is to set
C
TIMER
= 10nF and adjust its value accordingly to suit the
specific applications.
OPERATING SEQUENCE
Power-Up, Start-Up Check and Plug-In Timing Cycle
The sequence of operation for the LTC4211 is illustrated
in the timing diagram of Figure 5. When a PC board is
inserted into a live backplane, the LTC4211 first performs
Figure 5. Normal Power-Up Sequence
V
CC
ON
2μA 2μA
10μA
PLUG-IN CYCLE
FIRST TIMING CYCLE
200μA
POWER
GOOD
(V
FB
> V
REF
)
V
TMR
= V
REF
TIMER
12 345 6 7 8 9
RESET PULLED LOW DUE TO POWER BAD
10
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
RESET
POWER BAD
(V
FB
< V
REF
)
SOFT-START CYCLE
SECOND TIMING CYCLE
4211 F05
Table 1. t
TIMER
vs C
TIMER
C
TIMER
t
TIMER
0.0033μF 2.0ms
0.0047μF 2.9ms
0.0068μF 4.2ms
0.0082μF 5.1ms
0.01μF 6.2ms
0.015μF 9.3ms
0.022μF 13.6ms
0.033μF 20.4ms
0.047μF 29.0ms
0.068μF 42.0ms
0.082μF 50.7ms
0.1μF 61.8ms
0.15μF 92.7ms
0.22μF 136ms
0.33μF 204ms