Datasheet

MAX4271/MAX4272/MAX4273
3V to 12V Current-Limiting Hot-Swap Controllers
with Autoretry, DualSpeed/BiLevel Fault Protection
18 ______________________________________________________________________________________
Case B. Startup With Current Regulation
In applications where the board capacitance (C
BOARD
)
at V
OUT
is high, the inrush current causes a voltage
drop across R
SENSE
that exceeds the fast comparator
threshold (V
FC,TH
). In this case, the current charging
C
BOARD
can be considered constant and the turn-on
time is determined by:
t
ON
= C
BOARD
✕
V
IN
/ I
FAST,SET
where the maximum load current I
FAST,SET
= V
FC,TH
/
R
SENSE
. Figure 3 shows the waveforms and timing dia-
grams for a startup transient with current regulation. (See
Startup Time (C
BOARD
= 470µF) in the Typical Operating
Characteristics.) When operating under this condition, an
external gate capacitor is not required. Adding an exter-
nal gate capacitor at GATE to GND reduces the regulat-
ed current ripple but increases t
OFF
by increasing the
gate delay (t
d
) (Figure 4).
The actual startup time is determined by the longer of
the two timings of Case A and Case B. Set the startup
timer t
START
at 2
✕
t
ON
to guarantee enough time for
the output voltage to settle; also take into consideration
device parameter variation.
Retry
The retry timer defines the dead time before the IC tries
to restart a startup sequence following a fault detection.
This feature is available on the MAX4272/MAX4273.
Before selecting the retry timer value, determine how
long a temporary high-current fault condition may be
present. In the event of a permanent fault, the automat-
ic retry will effectively force current pulses through the
Figure 8. Startup and Retry Timeout
t
ON
AND t
RETRY
vs.
CTON AND CTIM
CAPACITANCE (nF)
t
ON
AND t
RETRY
(ms)
10,000
0.1
0.01
1
10
100
1000
0.01 10 1000.1 1 1000
t
RETRY
vs. CTIM (MAX4272/MAX4273)
t
ON
vs. CTIM (MAX4271/MAX4272)
t
ON
vs. CTON (MAX4273)
Figure 9. Operation with an External Capacitor
M1
C
SPD
*OPTIONAL (SEE TEXT)
C
TIM
CTIM
GATE
SENSE
GND
CSPD
ON
R
SENSE
V
OUT
C
GATE
C
BOARD
V
IN
IN
R
S*
MAX4271
MAX4272
MAX4273
R
PULLUP
STAT
Table 6. “No-Overcurrent” Turn-On Timing (Startup Without Current Limit)
MOSFET t
ON
(ms) MOSFET t
OFF
(ms)
DEVICE
C
GATE
(nF)
V
IN
= 3V V
IN
= 5V
V
IN
= 12V
V
IN
= 3V V
IN
= 5V
V
IN
= 12V
0 0.22 0.16 0.19 0.07 0.13 0.145
Fairchild FDS6670A
22 2.3 2 3.2 0.54 1.1 1.95
0 0.175 0.130 0.16 0.075 0.13 0.16
International Rectifier
IRF7401
22 1.9 1.8 3.5 0.54 1.1 2.0
0 0.101 0.074 0.073 0.033 0.067 0.085
Motorola MMSF5N03HD
22 2.0 1.8 3.2 0.470 1.0 1.95