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PDF LTC4251-2 Data sheet ( Hoja de datos )
| Número de pieza | LTC4251-2 | |
| Descripción | Negative Voltage Hot Swap Controllers | |
| Fabricantes | Linear Technology | |
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www.DataSheet4U.com
LTC4251/LTC4251-1/
LTC4251-2
Negative Voltage
Hot Swap Controllers in SOT-23
FEATURES
■ Allows Safe Board Insertion and Removal from a
Live –48V Backplane
■ Floating Topology Permits Very High Voltage
Operation
■ Programmable Analog Current Limit with Circuit
Breaker Timer
■ Fast Response Time Limits Peak Fault Current
■ Programmable Timer
■ Programmable Undervoltage/Overvoltage Protection
■ Low Profile (1mm) ThinSOTTM Package
U
APPLICATIO S
■ Hot Board Insertion
■ Electronic Circuit Breaker
■ –48V Distributed Power Systems
■ Negative Power Supply Control
■ Central Office Switching
■ Programmable Current Limiting Circuit
■ High Availability Servers
■ Disk Arrays
DESCRIPTIO
The LTC®4251/LTC4251-1/LTC4251-2 negative voltage
Hot SwapTM controllers allow a board to be safely inserted
and removed from a live backplane. Output current is
controlled by three stages of current limiting: a timed
circuit breaker, active current limiting and a fast feedforward
path that limits peak current under worst-case catastrophic
fault conditions.
Programmable undervoltage and overvoltage detectors
disconnect the load whenever the input supply exceeds
the desired operating range. The supply input is shunt
regulated, allowing safe operation with very high supply
voltages. A multifunction timer delays initial start-up and
controls the circuit breaker’s response time.
The LTC4251 UV/OV thresholds are designed to match the
standard telecom operating range of – 43V to – 75V. The
LTC4251-1 UV/OV thresholds extend the operating range
to encompass – 36V to – 72V. The LTC4251-2 implements
a UV threshold of – 43V only.
All parts are available in the 6-Pin SOT-23 package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
ThinSOT and Hot Swap are trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
–48V, 2.5A Hot Swap Controller
GND
GND
(SHORT PIN)
R1
402k
1%
R2
32.4k
1%
C1
10nF
RIN*
10k
500mW
3
VIN
56
UV/OV GATE
LTC4251
41
TIMER SENSE
VEE
CT 2
150nF
– 48V
*TWO 0.25W RESISTORS IN SERIES FOR
RIN ON THE PCB ARE RECOMMENDED.
CL +
100µF
CIN
1µF
LOAD
VOUT
Q1
IRF530S
RC
10Ω
CC
18nF
3
1
RS
0.02Ω
2
4 425112 TA01
GATE
5V/DIV
SENSE
2.5A/DIV
VOUT
20V/DIV
Start-Up Behavior
1ms/DIV
425112 TA02
425112fa
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1 page  
www.DataSheet4U.com
LTC4251/LTC4251-1/
LTC4251-2
TYPICAL PERFOR A CE CHARACTERISTICS UV/OV = 4V refers to UV = 4V for the LTC4251-2.
VGATE vs Temperature
14.5
UV/0V = 4V
14.0 VTMR = 0V
VSENSE = VEE
13.5
13.0
12.5
12.0
11.5
11.0
10.5
10.0
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
425112 G13
UV Threshold vs Temperature
2.55
2.50 LTC4251-1
2.45
VUVHI
2.40
2.35
2.30
2.25
2.20
VUVLO
2.15
2.10
2.05
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
425112 G16
ISENSE vs Temperature
–10
–12
–14
–16
–18
–20
–22
–24
–26
–28
–30
–55 –35 –15 5
UV/0V = 4V
TIMER = 0V
GATE = HIGH
VSENSE – VEE = 50mV
25 45 65 85 105 125
TEMPERATURE (°C)
425112 G19
VGATEL vs Temperature
0.8
UV/0V = 4V,
0.7 VTMR = 0V,
GATE THRESHOLD
0.6 BEFORE RAMP-UP
0.5
0.4
0.3
0.2
0.1
0
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
425112 G14
OV Threshold vs Temperature
6.45
LTC4251
6.25
VOVH
6.05
5.85
5.65
VOVL
5.45
5.25
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
425112 G17
ISENSE vs (VSENSE – VEE)
0.01
0.1
1.0
10
100
UV/0V = 4V
TIMER = 0V
GATE = HIGH
1000
TA = 25°C
–1.5 –1.0 –0.5 0 0.5 1.0 1.5 2.0
(VSENSE – VEE) (V)
425112 G20
UV Threshold vs Temperature
3.375
3.275
LTC4251/LTC4251-2
VUVH
3.175
3.075
2.975
2.875
VUVL
2.775
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
425112 G15
OV Threshold vs Temperature
6.51
6.41 LTC4251-1
6.31
6.21
VOVHI
6.11
6.01
5.91 VOVLO
5.81
5.71
5.61
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
425112 G18
TIMER Threshold vs Temperature
5.0
4.5
VTMRH
4.0
3.5
3.0
2.5
2.0
1.5
VTMRL
1.0
0.5
0
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
425112 G21
425112fa
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5 Page 
www.DataSheet4U.com
LTC4251/LTC4251-1/
LTC4251-2
APPLICATIO S I FOR ATIO
The R1-R2 divider values shown in the Typical Application
set a standing current of slightly more than 100µA, and
define an impedance at UV/OV of 30k. In most applica-
tions, 30k impedance coupled with 300mV UV hysteresis
makes the LTC4251/LTC4251-1/LTC4251-2 insensitive to
noise. If more noise immunity is desired, add a 1nF to 10nF
filter capacitor from UV/OV to VEE.
The UV and OV trip point thresholds for the LTC4251-1 are
designed to encompass the standard telecom operating
range of –36V to –72V.
A divider (R1, R2) is used to scale the supply voltage.
Using R1 = 442k and R2 = 34.8k gives a typical operating
range of 33.2V to 81V. The typical under- and overvoltage
shutdown thresholds are then 29.6V and 84.5V. 1%
divider resistors are recommended to preserve threshold
accuracy.
The R1-R2 divider values shown in the Typical Application
set a standing current of slightly more than 100µA, and
define an impedance at UV/OV of 32k. In most applica-
tions, 32k impedance coupled with 260mV UV hysteresis
makes the LTC4251-1 insensitive to noise. If more noise
immunity is desired, add a 1nF to 10nF filter capacitor from
UV/OV to VEE.
UV/OV OPERATION
A low input to the UV comparator will reset the chip and
pull the GATE and TIMER pins low. A low-to-high UV
transition will initiate an initial timing sequence if the three
remaining interlock conditions are met.
Overvoltage conditions detected by the OV comparator
will also pull GATE low, thereby shutting down the load,
but it will not reset the circuit breaker latch. Returning the
supply voltage to an acceptable range restarts the GATE
pin provided all interlock conditions except TIMER are
met.
TIMER
The operation of the TIMER pin is somewhat complex as
it handles several key functions. A capacitor, CT, is used at
TIMER to provide timing for the LTC4251/LTC4251-1/
LTC4251-2. Four different charging and discharging modes
are available at TIMER:
1. 5.8µA slow charge; initial timing delay
2. 230µA fast charge; circuit breaker delay
3. 5.8µA slow discharge; circuit breaker “cool-off”
4. Low impedance switch; resets capacitor after initial
timing delay, in undervoltage lockout, and in overvolt-
age
For initial startup, the 5.8µA pull-up is used. The low
impedance switch is turned off and the 5.8µA current
source is enabled when the four interlock conditions are
met. CT charges to 4V in a time period given by:
t = 4V • C T
5.8µA
(1)
When CT reaches 4V (VTMRH), the low impedance switch
turns on and discharges CT. The GATE output is enabled
and the load turns on.
CIRCUIT BREAKER TIMER OPERATION
If the SENSE pin detects more than 50mV across RS, the
TIMER pin charges CT with 230µA. If CT charges to 4V, the
GATE pin pulls low and the LTC4251/LTC4251-1/
LTC4251-2 latch off. The part remains latched off until
either the UV/OV pin is momentarily pulsed low, or VIN
dips into UVLO and is then restored. The circuit breaker
timeout period is given by
t = 4V • C T
230µA
(2)
Intermittent overloads may exceed the 50mV threshold at
SENSE, but if their duration is sufficiently short TIMER will
not reach 4V and the LTC4251/LTC4251-1/LTC4251-2
will not latch off. To handle this situation, the TIMER
discharges CT slowly with a 5.8µA pull-down whenever the
SENSE voltage is less than 50mV. Therefore any intermit-
tent overload with an aggregate duty cycle of 2.5% or more
will eventually trip the circuit breaker and latch off the
LTC4251/LTC4251-1/LTC4251-2. Figure 3 shows the cir-
cuit breaker response time in seconds normalized to 1µF.
The asymmetric charging and discharging of CT is a fair
gauge of MOSFET heating.
425112fa
11
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| PDF Descargar | [ Datasheet LTC4251-2.PDF ] | |
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