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:
1) 150ms after V
IN
exceeds the UVLO threshold (see
Over/Undervoltage Lockouts)
AND
2) 10µs after V
ON
>0.6V
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
MAX4271/MAX4272/MAX4273
3V to 12V Current-Limiting Hot-Swap Controllers
with Autoretry, DualSpeed/BiLevel Fault Protection
10 ______________________________________________________________________________________
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.
813ON
ON Comparator Input. Connect high for normal operation; connect low to force the
MOSFET off. Comparator threshold V
TH,ON
= 0.6V allows for precise control over
shutdown feature. Pulse ON low for 20μs 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.
