3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

______________________________________________________________________________________

19

has to be taken when choosing between immediate

retry and board space needed to manage the power

dissipation capabilities of the MOSFET (see Thermal

Considerations). The duty cycle is fixed to 1/32 for the

MAX4272, but can be varied in the MAX4273 by choos-

Additional External Gate Capacitance (CEXT)

An external gate capacitance can be connected at

GATE. This increases the time required to enhance the

MOSFET and further limits the output rise time. In the

MAX4271/MAX4272, connect the external capacitor

between GATE and GND. In the MAX4273, the external

capacitor can be connected between GATE and CEXT

or GND. If the capacitor is connected to CEXT, it is dis-

charged to ground during a slow comparator fault but it

is left floating during a fast comparator fault; this allows

the device to turn off the external MOSFET faster during

nonpolarized capacitor). Capacitance connected from

GATE to CEXT does little to decrease the regulated cur-

rent ripple. Add a small capacitor (5nF) from GATE to

graphs in the Typical Operating Characteristics.

Slow Comparator Response Time (CSPD)

The slow comparator threshold is set at 50mV, and its

response time is determined by the external capacitor

connected to CSPD (Figure 10).

A minimum response time of 20µs (typ) is achieved by

leaving this pin floating. This time is determined inter-

nally and is not affected by stray capacitance at CSPD

(up to 100pF).

Set the slow comparator response time to be longer

than the normal operation load transients (see Slow

Comparator).

ON and Reset Comparators

The ON comparator controls the ON/OFF function of

these devices. The ON comparator is a precision volt-

age comparator that can be used for temperature moni-

toring or as an additional UVLO (Figure 11). The

MAX4273 also features an uncommitted delayed com-

parator. This comparator can be used for voltage moni-

toring, power sequencing, or for generating a power-on

reset signal for on-card microprocessors (Figure 12).

0.6V with a 3mV (typ) hysteresis.

The uncommitted comparator OUTC output is an open-

drain output, and it is asserted low when its input volt-

age (INC) is below the threshold voltage. It goes into a

high-impedance state 150ms after the voltage has risen

above the threshold. The delay for negative-going

edges is 10µs.

Figure 13 shows the MAX4273 used to monitor precise-

ly the temperature of an external device such as the

MOSFET. This configuration uses the uncommitted

comparator to set the UVLO at a higher level by running

its output into the ON comparator’s input.

The ON comparator initiates startup when its input volt-

off the MOSFET when the voltage falls below the thresh-

old. The propagation delay is 10µs going high or low.

The ON comparator is also used to reset the

condition (see Latched/Autoretry).

Figure 10. Slow Comparator Response Time vs. CSPD

1000

0.01

0.1

1

10

100

1000

100

10

1

0.1

0.01

SLOW COMPARATOR

RESPONSE TIME vs. CSPD

CSPD (nF)

Figure 11. Temperature Monitoring and Protection

LOGIC

CONTROL

R2

ON

NTC

R1 = R2

R2 = VALUE OF THE NTC RESISTOR AT THE LIMIT TEMPERATURE

0.6V

R1

MAX4271

MAX4272

MAX4273