LTC4257CS8【PDF 16/20 页】IC CTRLR INTFACE PWR/ETHER 8SOIC

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Sheet目录42

12页 13页 14页 15页 [16页]17页 18页 19页 20页

LTC4257

4257fb

Load Capacitor

IEEE 802.3af requires that the PD maintain a minimum

load capacitance of 5礔. It is permissible to have a much

larger load capacitor and the LTC4257 can charge very

large load capacitors before thermal issues become a

problem. However, the load capacitor must not be too

large or the PD design may violate two IEEE 802.3af

requirements. The LTC4257 goes into current limit at

turn-on and charges the load capacitor with between

300mA and 400mA. The IEEE specification allows this

level of inrush current for up to 50ms. Therefore, it is

necessary that the PD complete charging of the capacitor

within the 50ms time limit. With a maximum input voltage

of 57V, these conditions limit the size of the load

capacitor to 250礔.

Very small output capacitors (d10礔) will charge very

quickly in current limit. The rapidly changing voltage at

the output may reduce the current limit temporarily,

causing the capacitor to charge at a somewhat reduced

rate. Conversely, charging very large capacitors may

cause the current limit to increase slightly. In either case,

once the output voltage reaches its final value, the input

current limit will be restored to its nominal value.

If the load capacitor is too large there can be an additional

problem with inadvertent power shutdown by the PSE.

Consider the following scenario. If the PSE is running at

APPLICATIO S I FOR ATIO

57V (maximum allowed) and the PD has been detected

and powered up, the load capacitor will be charged to

nearly 57V. If for some reason the PSE voltage suddenly

is reduced to 44V (minimum allowed), the input diodes

will reverse bias and PD power will be supplied solely by

the load capacitor. Depending on the size of the load

capacitor and the DC load of the PD, the PD will not draw

any power from the PSE for a period of time. If this period

of time exceeds the IEEE 802.3af 300ms disconnect

delay, the PSE may remove power from the PD. For this

reason, it is necessary to evaluate the load capacitance

and load current to ensure that inadvertent shutdown

cannot occur.

Maintain Power Signature

In an IEEE 802.3af system, the PSE uses the maintain

power signature (MPS) to determine if a PD continues to

require power. The MPS requires the PD to periodically

draw at least 10mA and also have an AC impedance less

than 26.25k& in parallel with 0.05礔. The PD application

circuits shown in this data sheet meet the requirements

necessary to maintain power. If either the DC current is

less than 10mA or the AC impedance is above 26.25k&,

the PSE might disconnect power. The DC current must be

less than 5mA and the AC impedance must be above 2M&

to guarantee power will be removed.

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