LTC4380HMS-1TRPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC4380HMS-1TRPBF Datasheet PDF : 20 Pages
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LTC4380
Applications Information
The LTC4380 limits the voltage and current delivered to
the load during supply transient or output overload events.
The N-channel MOSFET provides a low resistance path
from the input to the load during normal operation, while
in overvoltage conditions it limits the output to a threshold
voltage below the clamped gate voltage. The total fault timer
period is set to ride through short-duration faults, while
longer events cause the output to shut off and protect the
MOSFET from damage.
Startup
Figure 1 shows a 12V, 1A application which limits the
output to approximately 27V. When power is first ap-
plied with VCC ≥ 4V and ON ≥ 1.05V, there is a delay of
about 10ms before the GATE pin begins charging C2 and
Q1’s gate terminal with a fixed, 20µA current source. Q1,
operating as a source follower, ramps the output up at a
rate of IGATE(UP)/C2. Inrush current in the load capacitance
CL is given by
IINRUSH
=
IGATE(UP)
•
CL
C2
where IGATE(UP) is typically 20µA.
Eventually the GATE pin charges to the point where VIN ≈
VOUT and stops only when ΔVGATE (VGATE – VOUT) reaches
its regulation point of 11.5V, fully enhancing Q1.
Overcurrent Fault Protection
The LTC4380 features an adjustable current limit that
protects against short circuits and excessive load current.
During an overcurrent event, the GATE pin is regulated to
limit the current sense voltage across the SNS and OUT
pins (ΔVSNS) to 50mV when out is above 3V. In the case
of a severe short at the output, where OUT is less than
1.5V, the current sense voltage is 62mV. Output current is
thereby limited to ΔVSNS/RSNS. Current limit may control
the startup ramp rate in extreme cases, such as if CL is
unusually large or if current limit is set to an unusually low
value, and artificially reduces CL’s inrush current below
the value previously calculated.
Overvoltage Fault Protection
The LTC4380 limits the voltage at the output during an
overvoltage at the input. For the LTC4380-1/LTC4380-2
illustrated in Figure 1, an internal clamp limits the GATE
pin to either 31.5V or 50V, depending on the state of the
SEL pin. With the SEL pin grounded as shown, the GATE
pin is clamped at 31.5V. Assuming a threshold voltage of
5V for Q1, this limits the output to approximately 26.5V.
Tying the SEL pin high causes the GATE pin to clamp at
50V, and limits the output to approximately 45V.
The GATE pin may also be limited by the compliance of
the internal 20µA current source, to VCC + 13.5V. In the
LTC4380-3/LTC4380-4 the GATE pin clamp is entirely
disconnected, leaving only the VCC + 13.5V compliance
limit. This arrangement allows the GATE pin to be effectively
clamped at any voltage from 18V to 86V, by clamping VCC
to between 4V and 72V.
VCC Pin
The VCC pin operating range extends from 4V to 80V. In the
presence of an input overvoltage exceeding 80V, the VCC
pin must be protected by filtering or clamping. For short
duration spikes and transients exceeding 80V, filtering is
the most sensible means of protecting the VCC pin. R1 and
C1 provide filtering in Figure 1. Owing to the LTC4380’s low
ICC, values up to 20k may be used for R1 without seriously
impairing the lower end of the operating voltage range.
For long duration surges such as automotive load dump,
C1 becomes prohibitively large and Zener D1 is the most
VIN = 12V
Q1
FDB33N25
RSNS
20mΩ
RDRN
R3
10Ω
R2
33Ω
R1
249k
10k
C2
47nF
OUT
CL
22µF
C1
4.7µF
D1
68V
CMHZ5266B
DRN
VCC
ON
GATE
SNS OUT
LTC4380-2
GND
TMR
SEL
CTMR
8.2µF
4380 F01
Figure 1. 12V/1A, Output Limited to 27V
4380f
10
For more information www.linear.com/LTC4380
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