MAX4271/MAX4272/MAX4273
3V to 12V Current-Limiting Hot-Swap Controllers
with Autoretry, DualSpeed/BiLevel Fault Protection
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CTON determines the maximum time allowed to com-
plete turn-on for the MAX4273. The default values for
turn-on time (t
ON
) and t
RETRY
are chosen by leaving
these pins floating; they are 10µs and 320µs, respec-
tively. These are also the minimum values (not con-
trolled and dependent on stray capacitance). Longer
timings are determined by the size of the capacitor
according to Figure 8, and can be determined in Table
5, which lists the startup and retry timing parameters.
Set the t
ON
timer long enough to allow for the MOSFET to
be enhanced and the load capacitor to be charged com-
pletely .
There are two ways of completing the startup sequence.
Case A describes a startup sequence that does not use
the current-limiting feature and slowly turns on the MOS-
FET by limiting the gate dv/dt. Case B uses the current-
limiting feature and turns on the MOSFET as fast as
possible while still preventing a high inrush current.
Case A. Startup Without Current Regulation
There are three ways to turn on the MOSFET without
reaching the fast comparator current limit:
1) If the board capacitance is small, the inrush current
is low.
2) If the gate capacitance is high, the MOSFET turns on
slowly.
3) The fast comparator can be disabled (MAX4273
only).
In all three cases, t
ON
is determined only by the charge
required to enhance the MOSFET. Effectively, the small
gate-charging current limits the output voltage dv/dt.
This time can be extended by connecting an external
capacitor between GATE and GND (MAX4271/
MAX4272) (Figure 9) or between GATE and CEXT
(MAX4273). The turn-on time is dominated by the exter-
nal gate capacitance if this value is considerably higher
than the MOSFET gate capacitance. Table 6 shows the
timing required to enhance the recommended MOSFET
with or without the external capacitor; Figure 3 shows
the related waveforms and timing diagrams. (See Time
to Charge Gate vs. C
GATE
and Startup Time with
C
BOARD
= 0 in the Typical Operating Characteristics.)
Remember that a high gate capacitance also increases
the turn-off time (t
OFF
), except in the case of a
MAX4273 fast fault.
If an external gate capacitor is not used, R
S
is not nec-
essary. R
S
prevents MOSFET self-oscillations that can
occur when C
GATE
is high while C
BOARD
is low.
Electrical characteristics as specified by the manufac-
turer’s data sheet are:
FDS6670A: C
ISS
= 3200pF, Q
T
(MAX) = 50nC, R
DS(ON)
=
8.2mΩ
IRF7401: C
ISS
= 1600pF, Q
T
(MAX) = 48nC, R
DS(ON)
=
22mΩ
MMSF5N03HD: C
ISS
= 1200pF, Q
T
(MAX) = 21nC,
R
DS(ON)
= 40mΩ
Figure 7. Fast Comparator Threshold vs. RTH
