LTC4220 View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC4220 Datasheet PDF : 36 Pages
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LTC4252-1/LTC4252-2
LTC4252A-1/LTC4252A-2
APPLICATIONS INFORMATION
Analog Current Limit and Fast Current Limit
In Figure 17a, when SENSE exceeds VACL, GATE is
regulated by the analog current limit amplifier loop. When
SENSE drops below VACL, GATE is allowed to pull up. In
Figure 17b, when a severe fault occurs, SENSE exceeds
VFCL and GATE immediately pulls down until the analog
current amplifier establishes control. If the severe fault
causes VOUT to exceed VDRNCL, the DRAIN pin is clamped
at VDRNCL. IDRN flows into the DRAIN pin and is multiplied
by 8. This extra current is added to the TIMER pull-up
current of 230μA. This accelerated TIMER current of
[230μA+8 • IDRN] produces a shorter circuit breaker fault
delay. Careful selection of CT, RD and MOSFET can help
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
shown in Figure 18a. If a soft-start capacitor, CSS, is con-
nected to this SS pin, the soft-start response is modified
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 VTMRH to VTMRL
(time points 1 to 2) or by the OV pin falling below the
VOVLO 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
gm, the MOSFET current quickly reaches the soft-start
control value of VACL(t) (Equation 7). At time point 6, the
GATE voltage is controlled by the current limit amplifier.
The soft-start control voltage reaches the circuit breaker
voltage, VCB, at time point 7 and the circuit breaker TIMER
activates. As the load capacitor nears full charge, load
12
" t*DRN
TIMER
34
VTMRH
5.8μA
GATE
5.8μA
CB TIMES OUT
1
2
VTMRH
" t*DRN
TIMER
GATE
SS
SENSE
VOUT
DRAIN
PWRGD
VACL
VCB
SS
SENSE
VFCL
VACL VCB
VOUT
VDRNCL
DRAIN
PWRGD
425212 F17
28
(17a) Analog Current Limit Fault
(17b) Fast Current Limit Fault
Figure 17. Current Limit Behavior (All Waveforms Are Referenced to VEE)
425212fd
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