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参数资料
| 型号： | LTC4259CGW |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 电源管理 |
| 英文描述： | 4-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO36 |
| 封装： | 0.300 INCH, PLASTIC, SSOP-36 |
| 文件页数： | 22/32页 |
| 文件大小： | 324K |
| 代理商： | LTC4259CGW |
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LTC4259
29
4259i
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 CDET and CPSE capacitors is critical to
proper operation of the LTC4259’s AC disconnect sensing.
See the AC Disconnect section for more information. Also,
CPSE may be important to the voltage stability of a powered
port. Port voltage instability is generally not a problem if VEE,
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 CDET and CPSE as these have reduced volt-
age dependance while also being small and inexpensive.
Power Supplies
The LTC4259 must be supplied with 3.3V (VDD) and –48V
(VEE). Poor regulation on either of these supplies can lead
to noncompliance. The IEEE requires a PSE output voltage
between 44V and 57V. Allowing for voltage drop, mostly
across the AC blocking diodes DAC between VEE and the
PSE’s Ethernet jacks, means that VEE must be more
negative than –45V (more if DAC has more than 1V of
forward drop). When the LTC4259 begins powering an
Ethernet port, it controls the current through the port to
minimize disturbances on VEE. However if the VEE supply
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 VEE supply must
APPLICATIO S I FOR ATIO
WU
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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 VDD or VEE can adversely affect
detection, classification and the AC disconnection sens-
ing. In short, proper bypassing and stability of the VDD and
VEE supplies are important.
Another problem that can affect the VEE supply is insuffi-
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 VEE supply capable of less than 15.4W (number of
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 VEE supply capability should be at least a
few percent more than the maximum total power the PSE
will supply to PDs. Finally, the LTC4259’s draw current
from VEE. If the VDD supply is generated from VEE, that
power divided by the switcher efficiency must be added to
the VEE supply’s capability; (33mW ÷ efficiency [number
of PSE’s Ethernet ports]
÷4)typicallylessthan12mW/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 IACDMIN, tripping the DETECT pin
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
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| LTC4259CGW-ES | 制造商:LINEAR_TECH 功能描述: |
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| LTC4260CGN#PBF | 功能描述:IC CTLR HOT SWAP I2C 24-SSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 热交换 系列:- 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:119 系列:- 类型:热交换控制器 应用:通用型，PCI Express? 内部开关:无 电流限制:- 电源电压:3.3V，12V 工作温度:-40°C ~ 85°C 安装类型:表面贴装 封装/外壳:80-TQFP 供应商设备封装:80-TQFP（12x12） 包装:托盘 产品目录页面:1423 (CN2011-ZH PDF) |
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