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PDF LT4276 Data sheet ( Hoja de datos )
| Número de pieza | LT4276 | |
| Descripción | PD Forward/Flyback Controller | |
| Fabricantes | Linear | |
| Logotipo |  |
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LT4276
LTPoE++/PoE+/PoE
PD Forward/Flyback Controller
Features
n IEEE802.3af/at and LTPoE++™ 90W Powered Device
(PD) with Forward/Flyback Controller
n LT4276A Supports All of the Following Standards:
n LTPoE++ 38.7W, 52.7W, 70W and 90W
n IEEE 802.3at 25.5W Compliant
n IEEE 802.3af up to 13W Compliant
n LT4276B is IEEE 802.3at/af Compliant
n LT4276C is IEEE 802.3af Compliant
n Superior Surge Protection (100V Absolute Maximum)
n Wide Junction Temperature Range (–40°C to 125°C)
n Auxiliary Power Support as Low as 9V
n No Opto-Isolator Required for Flyback Operation
n External Hot Swap™ N-Channel MOSFET for Lowest
Power Dissipation and Highest System Efficiency
n >94% End-to-End Efficiency with LT4321 Ideal Bridge
n Available in a 28-Lead 4mm × 5mm QFN Package
Applications
n High Power Wireless Data Systems
n Outdoor Security Camera Equipment
n Commercial and Public Information Displays
n High Temperature Applications
Description
The LT®4276 is a pin-for-pin compatible family of IEEE
802.3 and LTPoE++ Powered Device (PD) controllers. It
includes an isolated switching regulator controller capable
of synchronous operation in both forward and flyback
topologies with auxiliary power support.
The LT4276A employs the LTPoE++ classification scheme,
receiving 38.7W, 52.7W, 70W or 90W of power at the PD
RJ45 connector, and is backwards compatible with IEEE
802.3. The LT4276B is a fully 802.3at compliant, 25.5W
Type 2 (PoE+) PD. The LT4276C is a fully 802.3af compli-
ant, 13W Type 1 (PoE) PD.
The LT4276 supports both forward and flyback power
supply topologies, configurable for a wide range of PoE
applications. The flyback topology supports No-Opto
feedback. Auxiliary input voltage can be accurately sensed
with just a resistor divider connected to the AUX pin.
The LT4276 utilizes an external, low RDS(ON) N-channel
MOSFET for the Hot Swap function, maximizing power
delivery and efficiency, reducing heat dissipation, and
easing the thermal design.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
LTPoE++, and Hot Swap are trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
Typical Application
+
AUX
37V-57V –
LTPoE++ 70W Power Supply in a Forward Mode
+
VPORT
–
0.1µF
3.3k
10nF
FMMT723
100µH
10µF
22µF • •
VPORT HS VIN HS SW
GATE
AUX
SRC VCC
LT4276A
RCLASS
RCLASS++
GND FB31 SS ROSC
VCC FFS
PG
DLY ISEN+
ISEN–
20mΩ
SG
0.1µF
10k 100pF
T2P ITHB
4276 TA01
100k
OPTO
TO MICROPROCESSOR
VCC
For more information www.linear.com/LT4276
+
+ 5V
13A
–
LT4276 Family
MAX DELIVERED
POWER
LTPoE++ 90W
LTPoE++ 70W
LTPoE++ 52.7W
LTPoE++ 38.7W
25.5W
13W
LT4276
GRADE
ABC
l
l
l
l
ll
lll
4276f
1
1 page  
Typical Performance Characteristics
Input Current vs Input Voltage
25k Detection Range
0.5
125°C
85°C
0.4
25°C
–40°C
0.3
0.2
0.1
0
0 2 4 6 8 10
VPORT VOLTAGE (V)
4276 G01
VFB31 vs Temperature
3.178
3.176
3.174
3.172
3.170
3.168
3.166
3.164
3.162
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
4276 G04
Current Sense Voltage
vs Duty Cycle, ITHB
160
140 VITHB = 0.96V (FB31 = 0V)
120 VITHB = 1.8V
100
80
60 VITHB = 2.3V
40
VITHB = 2.6V
20
VITHB = 2.9V
0
0 10 20 30 40 50 60 70
DUTY CYCLE (%)
4276 G07
Signature Resistance
vs Input Voltage
26.25
25.75
125°C
85°C
25°C
–40°C
25.25
24.75
24.25
23.75
123456789
VPORT VOLTAGE (V)
4276 G02
Feedback Amplifier Output Current
vs VFB31
15
125°C
85°C
10 25°C
–40°C
5
0
–5
–10
–15
2.57 2.77 2.97 3.17 3.37 3.57 3.77
FB31 VOLTAGE (V)
4276 G05
PG Delay Time vs Temperature in
Flyback Mode
350
300
250 RFFSSDLY = 52.3k
200
150
100
RFFSSDLY = 5.23k
50
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
4276 G08
For more information www.linear.com/LT4276
LT4276
VCC Current vs Temperature
12
10
300KHz
8
214KHz
6
4
2
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
4276 G03
Switching Frequency
vs Temperature
325
ROSC = 45.3k
300
275
250
225 ROSC OPEN
200
175
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
4276 G06
PG Delay Time vs Temperature in
Forward Mode
400
350 TPGDELAY, RFFSDLY = 52.3k
300
TPGSG, RFFSDLY = 52.3k
250
200
150
100 TPGDELAY, RFFSDLY = 10.5k
50
0
–50 –25
TPGSG, RFFSDLY = 10.5k
0 25 50 75
TEMPERATURE (°C)
100 125
4276 G09
4276f
5
5 Page 
LT4276
Applications Information
less than VAUXT. Use the following equations to set VAUXON
and VAUXOFF via R1 and R2 in Figure 7. A capacitor up to
1000pF may be placed between the AUX pin and GND to
improve noise immunity.
VAUXON must be lower than VHSOFF.
PG
tPGDELAY
SG
tPGon
tPGSG
R1=
VAUXON − VAUXOFF
IAUXH
=
VAUXHYS
IAUXH
+
R2 =
R1
VAUXOFF
VAUXT
− 1
R1≥
VAUX(MAX) − VAUXT
1.4mA
VAUX
–
R1 LT4276
AUX
R2
GND
4276 F08a
4276 F07
Figure 8: PG and SG Relationship in Flyback Mode
•+
•
PG
Figure 7. AUX Threshold and Hysteresis Calculation
SWITCHING REGULATOR CONTROLLER OPERATION
The switching regulator controller portion of the LT4276
is a current mode controller capable of implementing
either a flyback or a forward power supply. When used
in flyback mode, no opto-isolator is required for feedback
because the output voltage is sensed via the transformer’s
third winding.
LT4276
ISEN+
ISEN–
GND FFSDLY SG
RFFSDLY
••
4276 F08
Figure 9: Example PG and SG Connections in Flyback Mode
Flyback Mode
The LT4276 is programmed into flyback mode by placing
a resistor RFFSDLY from the FFSDLY pin to GND. This resis-
tor must be in the range of 5.23kΩ to 52.3kΩ. If using a
potentiometer to adjust RFFSDLY, ensure the adjustment
of the potentiometer does not exceed 52.3kΩ.The value
of RFFSDLY determines tPGDELAY according to the following
equations:
tPGDELAY ≈ 2.69ns / kΩ •RFFSDLY + 30ns
tPGSG ≈ 20ns
The PG and SG relationships in flyback mode are shown
in Figure 8.
The SG pin must be connected to the secondary side
MOSFET through a gate drive transformer as shown in
Figure 9. Add a Schottky diode from PG to GND as shown
in Figure 9 to prevent PG from going negative.
Forward Mode
The LT4276 is programmed into forward mode by placing
a resistor RFFSDLY from the FFSDLY pin to VCC. The RFFSDLY
resistor must be in the range of 10.5kΩ to 52.3kΩ. If using
a potentiometer to adjust RFFSDLY ensure the adjustment
of the potentiometer does not exceed 52.3kΩ.
The value of RFFSDLY determines tPGDELAY and tPGSG ac-
cording to the following equations:
tPGDELAY ≈ 7.16ns/kΩ • RFFSDLY + 17ns
tPGSG ≈ 5.60ns/kΩ • RFFSDLY + 7.9ns
The PG and SG relationships in forward mode are shown
in Figure 10.
For more information www.linear.com/LT4276
4276f
11
11 Page | ||
| Páginas | Total 26 Páginas | |
| PDF Descargar | [ Datasheet LT4276.PDF ] | |
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