AN2783 datasheet(20/47 Pages) STMICROELECTRONICS

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PoE section

AN2783

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Programming inrush current limit resistor R38 on the PM8800 reference board is left open,

thus the limits are set to the default values.

8.5

DC current limit

The continuous current limitation is internally set at 440 mA, but it is possible to modify it by

connecting a resistor between DCCL and VSS. This limitation is active after setting nPGD,

but in case the selected value is lower than the default inrush current, it also applies during

the inrush current phase.

The formula to select the desired DC current is the following:

Equation 4

Figure 9.

DC current limit vs. RDC

The PM8800 useful programming range for the DC current limitation is between 150 mA and

800 mA. The practical resistor value ranges between 15 k

Ω and 75 kΩ.

Please note that the DC current limit is not linked to the inrush current limit, both limits can

be set independently of each other.

Different current limits occur at different voltage drops between VSS and GND regardless of

the PM8800 operative phase:

●

For a drop < 3 V, the DC limit occurs with a default value of 440 mA

●

For a drop > 3 V but < 15 V, the 3rd step of inrush current occurs with a default value of

440 mA

●

For a drop >15 V and < 30 V, the 2nd step of inrush current occurs with a default value

of 250 mA

●

For a drop > 30 V, the 1st step of inrush current occurs with a default value of 140 mA

We suggest putting the DC limit over the inrush current which allows avoiding an increase of

current limiting during protection phases.

Programming DC current limit resistor R37 on the PM8800 standard reference board is left

open, leaving the default value as the limit, while a 15 k

Ω is mounted on the high-power

version, putting the limit at 740 mA typ.

DC kΩ

[]

11200

DC mA

[]

-----------------------

100

200

300

400

500

600

700

800

900

RDC [kohm]

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