+48V Quad Hot-Swap Controllers For

Power-Over-LAN

16

______________________________________________________________________________________

threshold current, the open-circuit detector turns off the

switch after a 25.6ms delay, FAULT goes low and the

Port-OC flag is set for that switch. To clear the switch

latched-off condition, FAULT and Port-OC flags drive

the corresponding ON input low.

Overcurrent Faults

When an on switch current exceeds the current-limit

threshold, foldback circuitry activates and regulates the

switch current. When current limit lasts for longer than

set low, and the FAULT flag is set. If auto-retry is

auto-retry is disabled, the switch remains latched off,

and FAULT is low. Reset the latch and FAULT by dri-

ving corresponding ON_ low.

Applications Information

Considerations for circuit design include output capaci-

tor requirements, current-limit requirements, setting the

maximum on-time in current limit, and choosing a suit-

able MOSFET and on-time duty cycle in auto-retry.

Output Capacitor Requirements

The load capacitor requirements should be determined

first, as this affects the required startup.

Current-Limit Requirements

The current limit should be set to at least 20% higher

than the expected full load current. If current limit is

also used to control startup current, then set this limit

high enough so that the output voltage can rise and

Ω to 40kΩ resistor

= 40k

Ω set according to the following equation:

Ω) (6.4ms).

startup condition. There are two cases:

1) For startup without current limit, when

the startup current does not reach the maximum cur-

value, but large enough to allow the switch to remain

on during large output load-current transients. The

turn off the external FET in case of output overload

or short-circuit condition.

2) For startup with current limit, when

which is expected when

Choosing Power MOSFET

The FET must withstand a short-circuit condition where

must have sufficient thermal capacitance to prevent

Choose Duty Cycle (setting DC)

The duty cycle can be adjusted to allow time for heat to

MOSFETs with lower thermal capacitance. For smaller

duty cycle, a smaller FET is sufficient. See Table 2 for

setting the duty cycle.

The auto-retry off-time should not be too long to keep

system wait time during retry period to a reasonable

Application Circuits

In a typical LAN system there are two ways to deliver

power over the LAN cable. Power can be supplied to

the unused cable pairs, or power can be supplied over

the signal pairs (Figures 9 and 10).

tC

II

C

VV

II

OL

MAX

L

L

CC

MAX

L

=

−

+

−

−

18

2

3

18

C

C

L

GATE

II

C

C

I

V

R

GATE

L

GATE

L

SC

SENSE

=+









≥

II

C

C

I

V

R

GATE

L

GATE

L

SC

SENSE

=+ <