3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

______________________________________________________________________________________

15

is stable. Any input voltage transient at IN below the

UVLO threshold will reset the device and initiate a start-

up sequence.

These devices also have an overvoltage lockout

(OVLO) feature that prevents the device from restarting

after a fault condition if the discharge has not been

Additionally, the MAX4273 LLMON pin discharges the

load line with a 1k

Ω pulldown and prevents startup until

the load voltage is below 0.1V.

Since the MAX4271/MAX4272 do not monitor the output

voltage, a startup sequence can be initiated while the

board capacitance is still charged.

A large board capacitance or a short startup period may

prevent the MAX4272 from charging completely in one

startup period. The MAX4272 responds to these condi-

tions by charging the capacitor with bursts defined by a

ing will be complete after several retries unless the resis-

tive load or current load excessively discharges the

board capacitance during the retry timeout. This feature

applies to the MAX4273 if LLMON is left floating or is

connected to GND. To prevent multiple charging bursts,

required to complete the charge of the board capaci-

tance (see Component Selection).

Gate Overvoltage Protection

New-generation MOSFETs have an absolute maximum

protect these MOSFETs, the MAX4271/MAX4272 limit the

gate-to-drain voltage (the MAX4273 limits the gate-to-

source voltage) to +7.5V with an internal zener diode. No

MAX4272). If GATE can be discharged to ground faster

than the output voltage, an external small-signal protec-

tion diode (D1) can be used, as shown in Figure 6. The

MAX4273 has the protection diode internal.

Applications Information

Component Selection

N-Channel MOSFET

Select the external N-channel MOSFET according to

the application’s current level. Table 2 lists some rec-

ommended components. The MOSFET’s on-resistance

imum voltage drop at full load to limit the MOSFET

ple if the board has pulsing loads or triggers an exter-

nal undervoltage reset monitor at full load. Determine

the device power-rating requirement to accommodate

a short circuit on the board at startup with the device

Figure 5. Status (STAT) Output Timing Diagram

O

1.2V

O

O

STAT

CTIM

(CTON)*

ON

FAULT CONDITION,

OR ON FALLING

EDGE

NO FAULT CONDITIONS PRESENT

*MAX4273 ONLY

Table 2. Recommended N-Channel

MOSFETs

PART

NUMBER

MANUFACTURER

DESCRIPTION

IRF7413

11m

Ω, 8 SO, 30V

IRF7401

22m

Ω, 8 SO, 20V

IRL3502S

International

Rectifier

6m

Ω, D2PAK, 20V

MMSF3300

20m

Ω, 8 SO, 30V

MMSF5N02H

30m

Ω, 8 SO, 20V

MTB60N05H

Motorola

14m

Ω, D2PAK, 50V

FDS6670A

10m

Ω, 8 SO, 30V

NDS8426A

13.5m

Ω, 8 SO, 20V

FDB8030L

Fairchild

4.5m

Ω, D2PAK, 30V

Figure 6. External Gate-Source Protection

D1

GATE

IN

SENSE

M1

N

MAX4271

MAX4272

GATE DRIVE

CHARGE PUMP