Current-Regulating Hot-Swap Controller with

DualSpeed/BiLevel Fault Protection

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11

__________Applications Information

Component Selection

N-Channel MOSFET

Select the external N-channel MOSFET according to

should be chosen low enough to have a minimum volt-

age drop at full load to limit the MOSFET power dissipa-

has pulsing loads, or it can trigger an external under-

voltage reset monitor at full load. Determine the

device’s power rating requirement to accommodate a

short-circuit condition on the board during start-up (see

MOSFET Thermal Considerations).

MOSFETs can typically withstand single-shot pulses

with higher dissipation than the specified package rat-

ing. Also, since part of the inrush current limiting is

achieved by limiting the gate dV/dt, it is not necessary

to use a MOSFET with low gate capacitance. Table 2

lists some recommended manufacturers and compo-

nents.

Sense Resistor

The slow comparator threshold voltage is set at 50mV.

Select a sense resistor that causes a 50mV voltage

drop at a current level above the maximum normal

operating current. Typically, set the overload current at

1.2 to 1.5 times the nominal load current. The fast com-

parator threshold is set at 200mV. This sets the fault

current limit at four times the overload current limit.

Choose the sense-resistor power rating to accommo-

date the overload current (Table 3):

tor connected at CTIM. This determines the maximum

time allowed to completely turn on the MOSFET.

floating and is approximately 5.5µs. This is also the

minimum value (not controlled and dependent on stray

capacitance). Longer timings are determined by the

value of the capacitor, according to Figure 6, and can

be determined as follows:

enhanced and the load capacitor to be completely

charged.

There are two methods of completing the start-up

sequences. Case A describes a start-up sequence that

does not use the current-limiting feature and slowly turns

on the MOSFET by limiting the gate dV/dt. Case B uses

the current-limiting feature and turns on the MOSFET as

fast as possible while still preventing high inrush current.

0.01

0.1

1

10

100

1000

CAPACITANCE (nF)

1000

0.1

0.01

0.001

1

10

100

1

50

0.5

100

(m

Ω)

5

10

OVERLOAD

THRESHOLD SET BY

SLOW COMPARATOR

(A)

4

2

20

FAULT CURRENT

THRESHOLD SET BY

FAST COMPARATOR

(A)

Table 2. Component Manufacturers

704-264-8861

888-522-5372

402-564-3131

PHONE

www.irctt.co

www.fairchildsemi.com

www.vishay.com

INTERNET

310-322-3331

IRC

Sense Resistors

602-244-3576

www.irf.com

www.mot-sps.com/ppd/

International Rectifier

Motorola

Fairchild

COMPONENT

Dale-Vishay

MANUFACTURER

MOSFETs