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參數資料
| 型號： | LTC4268IDKD-1#PBF |
| 廠商： | Linear Technology |
| 文件頁數： | 29/46頁 |
| 文件大小： | 419K |
| 描述： | IC PD HIGH POWER W/CNTRL 32-DFN |
| 產品培訓模塊： | Power over Ethernet |
| 標準包裝： | 52 |
| 類型： | 以太網供電開關（PoE） |
| 應用： | 遠程外設（工業控制，相機，數據訪問） |
| 內部開關： | 是 |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 32-WFDFN 裸露焊盤 |
| 供應商設備封裝： | 32-DFN（7x4）裸露焊盤 |
| 包裝： | 管件 |
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LTC4268-1
29
applicaTions inForMaTion
In general, better performance is obtained with a lower
turns ratio. A DC of 45.5% yields a 1:8 ratio. Note the
use of the external feedback resistive divider ratio to set
output voltage provides the user additional freedom in
selecting a suitable transformer turns ratio. Turns ratios
that are the simple ratios of small integers; e.g., 1:1, 2:1,
3:2 help facilitate transformer construction and improve
performance. When building a supply with multiple
outputs derived through a multiple winding transformer,
lower duty cycle can improve cross regulation by keeping
the synchronous rectifier on longer, and thus, keep
secondary windings coupled longer. For a multiple output
transformer, the turns ratio between output windings is
critical and affects the accuracy of the voltages. The ratio
between two output voltages is set with the formula V
OUT2
= V
OUT1
" N21 where N21 is the turns ratio between the
two windings. Also keep the secondary MOSFET R
DS(ON)
small to improve cross regulation. The feedback winding
usually provides both the feedback voltage and power for
the LTC4268-1. Set the turns ratio between the output and
feedback winding to provide a rectified voltage that under
worst-case conditions is greater than the 11V maximum
V
CC
turn-off voltage.
N
SF
>
V
OUT
11+ V
F
where:
V
F
=Diode Forward Voltage
For our example: N
SF
>
5
11+0.7
=
1
2.34
We will choose
1
Leakage Inductance
Transformer leakage inductance (on either the primary or
secondary) causes a spike after the primary side switch
turn-off. This is increasingly prominent at higher load
currents, where more stored energy is dissipated. Higher
flyback voltage may break down the MOSFET switch if it
has too low a BV
DSS
rating. One solution to reducing this
spike is to use a snubber circuit to suppress the voltage
excursion. However, suppressing the voltage extends the
flyback pulse width. If the flyback pulse extends beyond
the enable delay time, output voltage regulation is affected.
The feedback system has a deliberately limited input range,
roughly ?0mV referred to the FB node. This rejects higher
voltage leakage spikes because once a leakage spike is
several volts in amplitude; a further increase in amplitude
has little effect on the feedback system. Therefore, it is
advisable to arrange the snubber circuit to clamp at as
high a voltage as possible, observing MOSFET breakdown,
such that leakage spike duration is as short as possible.
Application Note 19 provides a good reference on snubber
design.
As a rough guide, leakage inductance of several percent
(of mutual inductance) or less may require a snubber, but
exhibit little to no regulation error due to leakage spike
behavior. Inductances from several percent up to perhaps
ten percent cause increasing regulation error.
Avoid double digit percentage leakage inductances. There
is a potential for abrupt loss of control at high load current.
This curious condition potentially occurs when the leakage
spike becomes such a large portion of the flyback waveform
that the processing circuitry is fooled into thinking that
the leakage spike itself is the real flyback signal! It then
reverts to a potentially stable state whereby the top of the
leakage spike is the control point, and the trailing edge of
the leakage spike triggers the collapse detect circuitry. This
typically reduces the output voltage abruptly to a fraction,
roughly one-third to two-thirds of its correct value. Once
load current is reduced sufficiently, the system snaps
back to normal operation. When using transformers with
considerable leakage inductance, exercise this worst-case
check for potential bistability:
1. Operate the prototype supply at maximum expected
load current.
2. Temporarily short-circuit the output.
3. Observe that normal operation is restored.
If the output voltage is found to hang up at an abnormally
low value, the system has a problem. This is usually evident
by simultaneously viewing the primary side MOSFET drain
voltage to observe firsthand the leakage spike behavior.
A final notethe susceptibility of the system to bistable
behavior is somewhat a function of the load current/
voltage characteristics. A load with resistivei.e., I = V/R
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相關代理商/技術參數 |
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| LTC4268IDKD-1-TR | 制造商:LINER 制造商全稱:Linear Technology 功能描述:High Power PD with Synchronous NoOpto Flyback Controller |
| LTC4268IDKD-1-TRPBF | 制造商:LINER 制造商全稱:Linear Technology 功能描述:High Power PD with Synchronous NoOpto Flyback Controller |
| LTC4269-1 | 制造商:LINER 制造商全稱:Linear Technology 功能描述:IEEE 802.3at PD with Synchronous No-Opto Flyback Controller |
| LTC4269-2 | 制造商:LINER 制造商全稱:Linear Technology 功能描述:IEEE 802.3at High Power PD and Synchronous Forward Controller with AUX Support |
| LTC4269CDKD-1#PBF | 功能描述:IC PD/OPTO FLYBACK CTRLR 32-DFN RoHS:是 類別:集成電路 (IC) >> PMIC - 熱交換 系列:- 標準包裝:50 系列:- 類型:熱交換控制器 應用:-48V 遠程電力系統，AdvancedTCA ? 系統，高可用性 內部開關:無 電流限制:可調 電源電壓:11.5 V ~ 14.5 V 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:10-TFSOP，10-MSOP（0.118"，3.00mm 寬） 供應商設備封裝:10-MSOP 包裝:管件 |
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