LTC4259CGW Linear Technology, LTC4259CGW Datasheet - Page 27

• Manufacturer Part Number: LTC4259CGW
• Manufacturer: Linear Technology

Specifications of LTC4259CGW

• Linear Misc Type: Negative Voltage
• Operating Supply Voltage (typ): -48V
• Operating Supply Voltage (max): -57V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 36
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIO S I FOR ATIO
does not implement these older detection methods auto-
matically, if software or external circuitry can detect the
noncompliant devices, the host controller may command
the LTC4259 to power the port, bypassing IEEE compliant
detection and classification and sending power to the
noncompliant device.
LOGIC LEVEL SUPPLY
Aside from the 48V used to source power down Ethernet
cables connected to each port, a logic level supply is
required to power the digital portion of the LTC4259. To
simplify design and meet voltage isolation requirements,
the logic level supply can be generated from the isolated
– 48V supply. Figure 19 shows an example method using
an LT
supply. This boost converter topology uses the LT1619
current mode controller and a current mirror which
reflects the 3.3V output voltage to the –48V rail, improv-
ing the regulation tolerance over the more traditional
large resistor voltage divider. This approach achieves
ISOLATED
®
1619 to control a –48V to 3.3V current mode
ISOLATED
–48V
GND
V
EE
+
CMPZ4702B
U
C12
2.2 F
100V
D17
U
R39
910k
W
R38
510
Q7
Si2328DS
C11
10 F
16V
Figure 19. –48V to 3.3V Boost Converter
S/S
V
IN
U
8
1
LT1619
GND
DRV
7
4
SENSE
GATE
V
FB
C
6
3
R32
47k
C7
4700pF
5
2
100 H
100
high accuracy with a transformerless design yielding
high performance (tight regulation).
IEEE 802.3af COMPLIANCE
The LTC4259 is designed to control power delivery in IEEE
802.3af compliant Power Sourcing Equipment (PSE).
Because proper operation of the LTC4259 may depend on
external signals and power sources, like the –48V supply
(V
such as the sense resistors (R
running on an external microprocessor, using the LTC4259
in a PSE does not guarantee 802.3af compliance. Using an
LTC4259 does get you most of the way there. This section
discusses the rest of the elements that go along with the
LTC4259 to make an 802.3af complaint PSE. Each para-
graph below addresses a component which is critical for
PSE compliance as well as possible pitfalls that can cause
a PSE to be noncompliant. For further assistance please
contact Linear Technology’s Applications department.
R37
L4
EE
C8
100pF
) or the OSCIN oscillator source, external components
R33
0.100
1%
1W
Q8
Si2328DS
FMMT593
B1100
D16
Q5
R36
3.32k
1%
R34
1.24k
1%
+
Q6
FMMT593
4259 F19
R35
100k
C9
47 F
10V
4.7 H
L3
S
), and possibly software
+
ISOLATED
GND
C10
47 F
10V
LTC4259
V
3.3V
300mA
DD
27
4259i