LTC4252-1/LTC4252-2
LTC4252A-1/LTC4252A-2
29
425212fe
For more information www.linear.com/LTC4252-1
applicaTions inForMaTion
current begins to decline below V
ACL
(t). The current limit
loop shuts off and GATE releases at time point 8. At time
point?, the SENSE voltage falls below V
CB
and TIMER
deactivates.
Large values of C
SS
can cause premature circuit breaker
time out as V
ACL
(t) may exceed the V
CB
potential during
the circuit breaker delay. The load capacitor is unable to
achieve full charge in one GATE start-up cycle. A more
serious side effect of large C
SS
values is SOA duration
may be exceeded during soft-start into a low impedance
load. A soft-start voltage below V
CB
will not activate the
circuit breaker TIMER.
Power Limit Circuit Breaker
Figure 19 shows the LTC4252A-1 in a power limit circuit
breaking application. The SENSE pin is modulated by the
board supply voltage, V
SUPPLY
. The D1 Zener voltage, V
Z
is set to be the same as the low supply operating voltage,
V
SUPPLY(MIN)
= 43V . If the goal is to have the high supply
operating voltage, V
SUPPLY(MAX)
= 71V giving the same
power at V
SUPPLY(MIN)
, then resistors R4 and R6 are
selected using the ratio:
R6
R4
=
V
CB
V
SUPPLY(MAX)
(16)
If R6 is 27? R4 is 38.3k. The peak circuit breaker power
limit is:
POWER
MAX
=
V
SUPPLY(MIN)
+V
SUPPLY(MAX)
(
)
2
4"V
SUPPLY(MIN)
"V
SUPPLY(MAX)
"POWER
SUPPLY(MIN)
=1.064"POWER
SUPPLY(MIN)
(17)
when
  V
SUPPLY
= 0.5 " (V
SUPPLY(MIN)
+ V
SUPPLY(MAX)
) = 57V .
The peak power at the fault current limit occurs at the supply
overvoltage threshold. The fault current limited power is:
OWER
FAULT
=
V
SUPPLY
R
S
"  V
ACL
  V
SUPPLY
V
Z
(
)
"
R6
R4
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
(18)
TIMER
GATE
SENSE
SS
DRAIN
V
TMRH
V
DRNCL
V
ACL
V
CB
V
DRNL
V
GS(th)
V
IN
V
GATEH
V
TMRL
425212 F18
PWRGD
5.8礎
58礎
58礎
TIMER
GATE
SENSE
SS
DRAIN
V
TMRH
V
DRNCL
V
CB
V
ACL
V
DRNL
V
GS(th)
V
IN
V
GATEH
V
TMRL
PWRGD
5.8礎
58礎
58礎
12 34 567    7a
8  9   10  11
END OF INTIAL TIMING CYCLE
12 34  5 6
7
8  9   10  11
END OF INTIAL TIMING CYCLE
20 " V
OS
20 " (V
CB
+ V
OS
)
20 " (V
ACL
+ V
OS
)
20 " V
OS
20 " (V
CB
+ V
OS
)
20 " (V
ACL
+ V
OS
)
230礎 + 8 " I
DRN
230礎 + 8 " I
DRN
Figure 18. Soft-Start Timing (All Waveforms Are Referenced to V
EE
)
(18a) Without External C
SS
(18b) With External C
SS
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