Detailed Description

The MAX4271/MAX4272/MAX4273 are circuit breaker

ICs designed for hot-swap applications where a line

card is inserted into a live backplane. Normally, when a

line card is plugged into a live backplane, the card’s

discharged filter capacitors provide a low impedance

that can momentarily cause the main power supply to

collapse. The MAX4271/MAX4272/MAX4273 are

designed to reside either in the backplane or in the

removable card to provide inrush current limiting and

short-circuit protection. This is achieved by using an

external N-channel MOSFET, an external current-sense

resistor, and two on-chip comparators. Figures 1 and 2

show the functional diagrams.

The timing and voltage levels for several critical parame-

ters can be adjusted with external resistors, external

capacitors, or by pin strapping. The timing components

are optional; without them, the part is set to its nominal

values, as shown in the Electrical Characteristics. The

parameters that can be adjusted are:

• Current-limit threshold

• Slow comparator response time

• Startup timer

• Fast comparator threshold

• Autoretry timeout period (time the part is shut down

after an overcurrent event)

• Fault management (latched/autoretry)

• Current overload threshold

Startup Mode

CTIM (MAX4271/MAX4272) or CTON (MAX4273) sets

the startup period (see Startup and Retry Timers). The

startup period begins after three conditions are met:

Over/Undervoltage Lockouts)

AND

AND

3) The device is no longer in retry mode.

During startup, the slow comparator is disabled and the

inrush current can be limited in two ways:

1) Slow ramping of the current to the load by control-

ling the external MOSFET gate voltage

OR

2) Limiting the current to the load by regulating the volt-

age across the external current-sense resistor

3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

10

______________________________________________________________________________________

PIN

MAX4271

MAX4272

MAX4273

NAME

FUNCTION

6

11

CTIM

Startup and Retry Timers Input. Controls the startup time and the autoretry time (32x

startup time) in the MAX4272, only the autoretry time in the MAX4273, and only the

startup time in the MAX4271. Leave floating or connect the timing capacitor from CTIM

to GND. Connect to IN for latched mode (this prevents autoretry in MAX4273). See

Startup and Retry Timers section.

7

12

STAT

Status Output. High indicates startup completed with no fault (Table 1). STAT is an

open-drain output.

813

ON

ON Comparator Input. Connect high for normal operation; connect low to force the

shutdown feature. Pulse ON low for 20

µs min to unlatch after a fault (MAX4273 in

latched mode, or MAX4271). Negative pulses are ignored during autoretry (MAX4273

in autoretry mode, or MAX4272). See ON and Reset Comparators section.

—

14

REF

1.2V Reference Output. Do not bypass with a capacitor to GND.

—

15

INC

Uncommitted Comparator Input. Controls OUTC.

—

16

OUTC

Uncommitted Comparator Output. OUTC goes high 150ms after INC goes high. OUTC

goes low immediately after INC goes low. OUTC is an open-drain output.

Pin Description (continued)