Unlike other circuit breaker ICs, the MAX4271/

MAX4272/MAX4273 regulate the current to a preset

level instead of completely turning off the external

N-FET if an overcurrent condition occurs during startup.

In startup mode, the gate drive current is limited to

100µA and decreases with the increase of the gate

voltage (see Gate Charge Current vs. Gate Voltage in

the Typical Operating Characteristics). This allows the

controller to slowly enhance the MOSFET. If the fast

comparator detects an overcurrent, the gate voltage is

momentarily discharged with a fixed 70µA current until

decreases below its threshold point. This effectively

regulates the inrush current during startup. Figure 3

shows the startup waveforms. STAT goes high at the

end of the startup period if no fault condition is present.

Normal Operation (DualSpeed/BiLevel)

In normal operation (after the startup timer has

expired), protection is provided by turning off the exter-

nal MOSFET when a fault condition is encountered.

DualSpeed/BiLevel fault protection incorporates two

comparators with different thresholds and response

times to monitor the current:

1) Slow comparator. This comparator has an externally

set response time (from 20µs to seconds) and a fixed

50mV threshold voltage. The slow comparator

ignores low-amplitude momentary current glitches.

After an extended overcurrent condition, a fault is

generated and the MOSFET gate is slowly dis-

charged.

2) Fast comparator. This comparator has a fixed 350ns

response time and a 200mV threshold voltage

(adjustable from 50mV to 750mV in MAX4273). The

fast comparator turns off the MOSFET immediately

after it detects a large amplitude event such as a

short circuit.

3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

______________________________________________________________________________________

11

Figure 1. MAX4271/MAX4272 Functional Diagram

MAX4271

MAX4272

6

µA

SLOW COMPARATOR

FAST COMPARATOR

6

µA

2.45V

CSPD

INPUT

UVLO

LOGIC

CONTROL

DISCHARGE

ENABLE

STAT

GND

CTIM

4

µA

0.1V

0.6V

ON

N

M1

GATE

SENSE

IN

GATE

OVLO

ON COMPARATOR

GATE

DRIVE

50mV

200mV

150ms

DELAY

ON

CHARGE

PUMP

(1MHz)