3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

16

______________________________________________________________________________________

configured in automatic retry mode (see MOSFET

Thermal Considerations).

Using the MAX4271/MAX4273 in latched mode allows

lower power ratings. A MOSFET can typically withstand

single-shot pulses with higher dissipation than the

specified package rating. Low MOSFET gate capaci-

tance is not necessary since the inrush current limiting

is achieved by limiting the gate dv/dt. Table 3 lists

some recommended manufacturers and components.

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 typically set at 200mV. This will set

the fault current limit at four times the overload current

limit. The MAX4273 fast comparator threshold can be

set between 50mV and 750mV; see Table 4 for a

detailed listing.

Choose the sense resistor power rating according to

the device configuration. If no retry mode is selected,

Fast Comparator Threshold (RTH) (MAX4273)

The fast comparator threshold is determined by the

external resistor connected at RTH. To select threshold

voltages between 50mV and 750mV, use resistor val-

ues between 5k

Ω and 75kΩ according to Figure 7.

Resistor values between 200

Ω and 5kΩ are not recom-

mended. Setting the threshold voltage of the fast com-

parator below 50mV will effectively override the slow

comparator operation. The MAX4273 fast comparator

25mV or less). Ground RTH to set the threshold to

200mV internally.

Startup and Retry Timers (CTIM, CTON)

mined by the capacitors connected at CTIM and

CTON. The capacitor connected to CTIM has two func-

tions for the three devices as follows:

PART NUMBER

OVERLOAD THRESHOLD

SET BY SLOW COMPARATOR (A)

FAULT CURRENT THRESHOLD

SET BY FAST COMPARATOR (A)

MAX4271/MAX4272

5

20

10

MAX4273

5

5 to 75

MAX4271/MAX4272

1

4

50

MAX4273

1

1 to 15

MAX4271/MAX4272

0.5

2

100

MAX4273

0.5

0.5 to 7.5

Table 3. Component Manufacturers

COMPONENT

MANUFACTURER

PHONE

WEB

Dale-Vishay

402-564-3131

www.vishay.com

Sense Resistors

IRC

704-264-8861

www.irctt.com

Fairchild

888-522-5372

www.fairchildsemi.com

International Rectifier

310-322-3331

www.irf.com

MOSFETS

Motorola

602-244-3576

www.mot-sps.com/ppd/

DEVICE

CTIM FUNCTION

MAX4271

Startup time

MAX4272

Startup time and sets retry timer

MAX4273

Sets retry timer