LTC4259CGW Linear Technology, LTC4259CGW Datasheet - Page 29

• 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
handle dissipating at least 0.5W should not be stressed in
this application. Other component leakages can have a
similar affect on AC disconnect and affect DC disconnect
if the leakage becomes more severe. Among components
to be wary of are the transient surge suppressors. The
devices shown in Figure 1 are rated for less than 5 A of
leakage at 58V. However there is a potential for stress
induced leakage, so healthy margins should be used when
selecting diodes for these applications.
Capacitors
Sizing of both the C
proper operation of the LTC4259’s AC disconnect sensing.
See the AC Disconnect section for more information. Also,
C
port. Port voltage instability is generally not a problem if V
the –48V supply, is well bypassed. For both of these rea-
sons be aware that many ceramic dielectrics have dramatic
capacitance versus DC voltage coefficients. A 0.22 F ce-
ramic capacitor is often nowhere near 0.22 F when oper-
ating at 50VDC or 100VDC. Use 100V or higher rated X7R
capacitors for C
age dependance while also being small and inexpensive.
Power Supplies
The LTC4259 must be supplied with 3.3V (V
(V
to noncompliance. The IEEE requires a PSE output voltage
between 44V and 57V. Allowing for voltage drop, mostly
across the AC blocking diodes D
PSE’s Ethernet jacks, means that V
negative than –45V (more if D
forward drop). When the LTC4259 begins powering an
Ethernet port, it controls the current through the port to
minimize disturbances on V
is underdamped or otherwise unstable, its voltage could
go outside of the IEEE specified limits causing all ports in
the PSE to be noncompliant. This scenario can be even
worse when a PD is unplugged because the current drops
immediately to zero. In both cases the V
PSE
EE
). Poor regulation on either of these supplies can lead
may be important to the voltage stability of a powered
DET
U
and C
DET
and C
PSE
U
EE
as these have reduced volt-
PSE
. However if the V
AC
AC
capacitors is critical to
W
has more than 1V of
between V
EE
EE
must be more
DD
supply must
) and –48V
U
EE
EE
and the
supply
EE
,
always stay between –45V and –57V. In addition, the
802.3af specification places specific ripple, noise and load
regulation requirements on the PSE. Among other things,
disturbances on either V
detection, classification and the AC disconnection sens-
ing. In short, proper bypassing and stability of the V
V
Another problem that can affect the V
cient power, leading to the supply voltage drooping out of
the specified range. The 802.3af specification states that
if a PSE powers a PD it must be able to provide the
maximum power level requested by the PD based on the
PD’s classification. The specification does allow a PSE to
choose not to power a port because the PD requires more
power than the PSE has left to deliver. If a PSE is built with
a V
PSE’s Ethernet ports), it must implement a power alloca-
tion algorithm that prevents ports from being powered
when there is insufficient power. Because the specifica-
tion also requires the PSE to supply 400mA at up to a 5%
duty cycle, the V
few percent more than the maximum total power the PSE
will supply to PDs. Finally, the LTC4259’s draw current
from V
power divided by the switcher efficiency must be added to
the V
of PSE’s Ethernet ports] 4) typically less than 12mW/port.
AC disconnect also relies on an oscillating signal applied
to the OSCIN pin. Requirements for this signal are pro-
vided in the OSCIN Input and Oscillator Requirements
section. Out-of-band noise on the OSCIN pin will disrupt
the LTC4259’s ability to sense the absence of a PD. Any
noise present at the OSCIN pin is amplified by the LTC4259
and driven out of the DETECT pins (of powered ports with
AC disconnect enabled). Due to the amount of capacitance
connected to the DETECT pins, driving this noise can
easily require more than I
current sense and keeping the port powered. During
circuit board layout, keep wiring from the oscillator to the
OSCIN pin away from noise sources like digital clock and
EE
EE
supplies are important.
EE
supply capable of less than 15.4W • (number of
EE
supply’s capability; (33mW efficiency • [number
. If the V
EE
DD
supply capability should be at least a
supply is generated from V
ACDMIN
DD
or V
, tripping the DETECT pin
EE
can adversely affect
EE
LTC4259
supply is insuffi-
29
EE
DD
, that
and
4259i