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PDF LT4256-1 Data sheet ( Hoja de datos )
| Número de pieza | LT4256-1 | |
| Descripción | (LT4256-1/-2) Positive High Voltage Hot Swap Controllers | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT4256-1/LT4256-2
Positive High Voltage
Hot Swap Controllers
FEATURES
■ Allows Safe Board Insertion and Removal from a
Live Backplane
■ Controls Supply Voltage from 10.8V to 80V
■ Foldback Current Limiting
■ Overcurrent Fault Detection
■ Drives an External N-Channel MOSFET
■ Programmable Supply Voltage Power-Up Rate
■ Undervoltage Protection
■ Latch Off Operation Mode (LT4256-1)
■ Automatic Retry (LT4256-2)
■ Available in an 8-Pin SO Package
U
APPLICATIO S
■ Hot Board Insertion
■ Electronic Circuit Breaker/Power Bussing
■ Industrial High Side Switch/Circuit Breaker
■ 24V/48V Industrial/Alarm Systems
■ Ideally Suited for 12V, 24V and 48V Distributed
Power Systems
■ 48V Telecom Systems
DESCRIPTIO
The LT®4256-1/LT4256-2 are high voltage Hot SwapTM
controllers that allow a board to be safely inserted and
removed from a live backplane. An internal driver drives an
external N-channel MOSFET switch to control supply
voltages ranging from 10.8V to 80V.
The LT4256-1/LT4256-2 features an adjustable analog
foldback current limit. If the supply remains in current limit
for more than a programmable time, the N-channel
MOSFET shuts off and the PWRGD output asserts low. The
LT4256-2 automatically restarts after a time-out delay.
The LT4256-1 latches off until the UV pin is cycled low.
The PWRGD output indicates when the output voltage
rises above a programmed level. An external resistor
string from VCC provides programmable undervoltage
protection.
The LT4256 can be used as an upgrade to LT1641 designs.
See Table 1 on page 14 for upgraded specifications.
The LT4256-1 and LT4256-2 are available in an 8-pin SO
package that is pin compatible with the LT1641.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Hot Swap is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATIO
48V, 2A Hot Swap Controller
VIN
0.02Ω
IRF530
48V SMAT70A
CMPZ5241B
(SHORT PIN)
87
11V
64.9k
VCC
1
UV
SENSE 10Ω
6
GATE
10nF
100Ω
36.5k
0.1µF
8.06k
LT4256-1/
LT4256-2
2
FB
4.02k
VOUT
48V
CL 2A
27k
VIN
50V/DIV
VOUT
50V/DIV
INRUSH
CURRENT
500mA/DIV
LT4256 Start-Up Behavior
5
TIMER
3
PWRGD
UV = 36V
PWRGD
4256 TA01
PWRGD CL = 225µF
50V/DIV
2.5ms/DIV
4256 TA02
GND
33nF GND
PWRGD = 40V
4
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1
1 page  
LT4256-1/LT4256-2
TYPICAL PERFOR A CE CHARACTERISTICS Specifications are at TA = 25°C unless
otherwise noted.
FB Pin Current vs FB Pin Voltage
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
0 10 20 30 40 50
VFB (V)
4256 G15
Gate Pull-Down Capability vs VCC
Below Minimum Operating Voltage
60
50
40
30
20
10
0
0 2 4 6 8 10 12
VCC (V)
4256 G16
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5 Page 
APPLICATIO S I FOR ATIO
LT4256-1/LT4256-2
IOUT
500mA/DIV
IOUT
500mA/DIV
TIMER
5V/DIV
TIMER
5V/DIV
VOUT
50V/DIV
VOUT
50V/DIV
GATE
50V/DIV
10ms/DIV
4256 F09
Figure 9. LT4256-1 Current Limit Waveforms
GATE
50V/DIV
10ms/DIV
4256 F10
Figure 10. LT4256-2 Current Limit Waveforms
When the TIMER pin reaches 4.65V (typ), the internal fault
latch is set causing GATE to be pulled low and TIMER to be
discharged to GND by the 3µA current source. The part is
not allowed to turn on again until the voltage on TIMER
falls below 0.65V (typ).
TIMER must never be pulled high by a low impedance
because whenever TIMER rises above the upper threshold
(typically 4.65V) the pin characteristics change from a
high impedance current source to a low impedance.
Whenever GATE is commanded off by any fault condition,
it is discharged rapidly, turning off the external MOSFET.
The waveform in Figure 9 shows how the output latches off
following a current fault (LT4256-1). The drop across the
sense resistor is held at 55mV as the timer ramps up. Once
TIMER reaches its shutdown threshold (4.65V typically),
the circuit latches off.
The LT4256-1 latches off after a current limit fault. After
the LT4256-1 latches off, the part may be commanded to
start back up. This is accomplished by cycling UV to
ground and then back high (this command can only be
accepted after TIMER discharges back below the 0.65V
typical threshold, to prevent overheating transistor Q1).
Automatic Restart
The LT4256-2 will automatically restart after an overcurrent
fault. These waveforms are shown in Figure 10.
The LT4256-2 functionality is as follows: When an
overcurrent condition occurs, the GATE pin is servoed to
maintain a constant voltage across the sense resistor, and
the capacitor C2 at the TIMER pin will begin to charge.
When the voltage at the TIMER pin reaches 4.65V (typ),
the GATE pin is pulled low. When the voltage at the TIMER
pin ramps back down to 0.65V (typ), the LT4256-2 turns
on again. If the short-circuit condition at the output still
exists, the cycle will repeat itself indefinitely. The duty
cycle under short-circuit conditions is 3% which prevents
Q1 from overheating.
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| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT4256-1.PDF ] | |
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