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| 型號： | LTC4278IDKD#PBF |
| 廠商： | Linear Technology |
| 文件頁數(shù)： | 22/42頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC PD IEEE 802.3AT 25.5W 32-DFN |
| 產(chǎn)品培訓模塊： | Power over Ethernet |
| 標準包裝： | 52 |
| 控制器類型： | 供電設(shè)備接口控制器（PD） |
| 接口： | IEEE 802.3af |
| 電源電壓： | 4.5 V ~ 20 V |
| 電流 - 電源： | 6.4mA |
| 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(shè)備封裝： | 32-DFN（7x4）裸露焊盤 |
| 包裝： | 管件 |
| 產(chǎn)品目錄頁面： | 1338 (CN2011-ZH PDF) |
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LTC4278
29
4278fc
APPLICATIONS INFORMATION
Selecting the Load Compensation Resistor
The expression for RCMP was derived in the Operation
section as:
RCMP =K1
RSENSE 1–DC
(
)
ESR
+RDS(ON)
R1NSF
Continuing the example:
K1=
VOUT
VIN Eff
=
5
48 90%
=
0.116
DC=
1
1+
NVIN(NOM)
VOUT
=
1
1+ 1
8
48
5
=
45.5%
If ESR+RDS(ON) =8mΩ
RCMP =0.116
33mΩ 10.455
(
)
8mΩ
37.4kΩ 1
3
=
3.25k
This value for RCMP is a good starting point, but empirical
methods are required for producing the best results.
This is because several of the required input variables
are difficult to estimate precisely. For instance, the ESR
term above includes that of the transformer secondary,
but its effective ESR value depends on high frequency
behavior, not simply DC winding resistance. Similarly, K1
appears as a simple ratio of VIN to VOUT times efficiency,
but theoretically estimating efficiency is not a simple
calculation.
The suggested empirical method is as follows:
1. Build a prototype of the desired supply including the
actual secondary components.
2. Temporarily ground the CCMP pin to disable the load
compensation function. Measure output voltage while
sweeping output current over the expected range.
Approximate the voltage variation as a straight line.
ΔVOUT/ΔIOUT = RS(OUT) .
3. Calculate a value for the K1 constant based on VIN, VOUT
and the measured efficiency.
4. Compute:
RCMP =K1
RSENSE
RS(OUT)
R1NSF
5. Verify this result by connecting a resistor of this value
from the RCMP pin to ground.
6.DisconnectthegroundshorttoCCMPandconnecta0.1F
filter capacitor to ground. Measure the output imped-
anceRS(OUT)=ΔVOUT/ΔIOUTwiththenewcompensation
in place. RS(OUT) should have decreased significantly.
Fine tuning is accomplished experimentally by slightly
altering RCMP. A revised estimate for RCMP is:
′
RCMP =RCMP 1+
RS(OUT)CMP
RS(OUT)
where R
′CMP is the new value for the load compensation
resistor. RS(OUT)CMP is the output impedance with RCMP
in place and RS(OUT) is the output impedance with no
load compensation (from step 2).
Setting Frequency
The switching frequency of the LTC4278 is set by an
external capacitor connected between the OSC pin and
ground. Recommended values are between 200pF and
33pF, yielding switching frequencies between 50kHz and
250kHz.Figure12showsthenominalrelationshipbetween
external capacitance and switching frequency. Place the
capacitor as close as possible to the IC and minimize OSC
COSC (pF)
30
50
f OSC
(kHz)
100
200
300
100
200
4278 F12
Figure 12. fOSC vs OSC Capacitor Values
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