LTC4252-2 Linear Technology, LTC4252-2 Datasheet - Page 28

• Manufacturer Part Number: LTC4252-2
• Description: Negative Voltage Hot Swap Controllers
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
LTC4252-1/LTC4252-2
LTC4252A-1/LTC4252A-2
Analog Current Limit and Fast Current Limit
In Figure 17a, when SENSE exceeds V
lated by the analog current limit amplifier loop. When
SENSE drops below V
Figure 17b, when a severe fault occurs, SENSE exceeds
V
current amplifier establishes control. If the severe fault
causes V
at V
by 8. This extra current is added to the TIMER pull-up
current of 230µA. This accelerated TIMER current of
[230µA+8 • I
delay. Careful selection of C
prevent SOA damage in a low impedance fault condition.
Soft-Start
If the SS pin is not connected, this pin defaults to a linear
voltage ramp, from 0V to 2.2V in about 180µs (or 0V to
1.4V in 230µs for the LTC4252A) at GATE start-up, as
28
FCL
DRNCL
and GATE immediately pulls down until the analog
OUT
. I
DRN
230µA + 8 • I
DRN
to exceed V
PWRGD
SENSE
TIMER
DRAIN
flows into the DRAIN pin and is multiplied
GATE
] produces a shorter circuit breaker fault
V
OUT
SS
U
DRN
ACL
1
(17a) Analog Current Limit Fault
2
DRNCL
, GATE is allowed to pull up. In
U
Figure 17. Current Limit Behavior (All Waveforms are Referenced to V
3
T
4
, R
, the DRAIN pin is clamped
V
V
V
D
TMRH
ACL
CB
and MOSFET can help
W
ACL
5.8µA
, GATE is regu-
U
5.8µA
shown in Figure 18a. If a soft-start capacitor, C
connected to this SS pin, the soft-start response is modi-
fied from a linear ramp to an RC response (Equation 6), as
shown in Figure 18b. This feature allows load current to
slowly ramp-up at GATE start-up. Soft-start is initiated at
time point 3 by a TIMER transition from V
(time points 1 to 2) or by the OV pin falling below the V
threshold after an OV condition. When the SS pin is below
0.2V, the analog current limit amplifier holds GATE low.
Above 0.2V, GATE is released and 58µA ramps up the
compensation network and GATE capacitance at time
point 4. Meanwhile, the SS pin voltage continues to ramp
up. When GATE reaches the MOSFET’s threshold, the
MOSFET begins to conduct. Due to the MOSFET’s high g
the MOSFET current quickly reaches the soft-start control
value of V
voltage is controlled by the current limit amplifier. The
soft-start control voltage reaches the circuit breaker volt-
age, V
activates. As the load capacitor nears full charge, load
230µA + 8 • I
PWRGD
TIMER
SENSE
DRAIN
GATE
V
CB
OUT
V
SS
V
DRNCL
, at time point 7 and the circuit breaker TIMER
TMRH
DRN
ACL
1
(t) (Equation 7). At time point 6, the GATE
V
(17b) Fast Current Limit Fault
FCL
CB TIMES OUT
2
V
ACL
V
CB
EE
)
4252-1/2 F17
TMRH
to V
SS
425212fb
TMRL
OVLO
, is
m
,