Datasheet

MAX4271/MAX4272/MAX4273
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
completed. V
GATE
has to be discharged to below 0.1V.
Additionally, the MAX4273 LLMON pin discharges the
load line with a 1kΩ 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
t
ON
duty cycle and a period of t
ON
+ t
RETRY
. The charg-
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,
ensure that the t
ON
timer exceeds the minimum time
required to complete the charge of the board capaci-
tance (see Component Selection).
Gate Overvoltage Protection
New-generation MOSFETs have an absolute maximum
rating of ±8V for the gate-to-source voltage (V
GS
). To
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
protection is provided for negative V
GS
(MAX4271/
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
(R
DS(ON)
) should be chosen low enough to have a min-
imum voltage drop at full load to limit the MOSFET
power dissipation. High R
DS(ON)
can cause output rip-
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
t
START
O
O
STAT
CTIM
(CTON)*
ON
FAULT CONDITION,
OR ON FALLING
EDGE
NO FAULT CONDITIONS PRESENT
*MAX4273 ONLY
V
IN
V
IN
Table 2. Recommended N-Channel
MOSFETs
PART
NUMBER
MANUFACTURER
DESCRIPTION
IRF7413 11mΩ, 8 SO, 30V
IRF7401 22mΩ, 8 SO, 20V
IRL3502S
International
Rectifier
6mΩ, D2PAK, 20V
MMSF3300 20mΩ, 8 SO, 30V
MMSF5N02H
30mΩ, 8 SO, 20V
MTB60N05H
Motorola
14mΩ, D2PAK, 50V
FDS6670A 10mΩ, 8 SO, 30V
NDS8426A 13.5mΩ, 8 SO, 20V
FDB8030L
Fairchild
4.5mΩ, D2PAK, 30V
Figure 6. External Gate-Source Protection
D1
GATE
R
SENSE
V
OUT
V
GD
V
GS
C
BOARD
IN
SENSE
M1
N
MAX4271
MAX4272
GATE DRIVE
CHARGE PUMP