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3V to 12V Current-Limiting Hot-Swap Controllers
with Autoretry, DualSpeed/BiLevel Fault Protection
Table 6. “No-Overcurrent” Turn-On Timing (Startup Without Current Limit)
DEVICE
Fairchild FDS6670A
International Rectifier
IRF7401
Motorola MMSF5N03HD
C GATE
(nF)
0
22
0
22
0
22
V IN = 3V
0.22
2.3
0.175
1.9
0.101
2.0
MOSFET t ON (ms)
V IN = 5V
0.16
2
0.130
1.8
0.074
1.8
V IN = 12V
0.19
3.2
0.16
3.5
0.073
3.2
V IN = 3V
0.07
0.54
0.075
0.54
0.033
0.470
MOSFET t OFF (ms)
V IN = 5V
0.13
1.1
0.13
1.1
0.067
1.0
V IN = 12V
0.145
1.95
0.16
2.0
0.085
1.95
t ON AND t RETRY vs.
V IN
R SENSE
M1
V OUT
CTON AND CTIM
10,000
1000
t RETRY vs. CTIM (MAX4272/MAX4273)
R PULLUP
R S *
C BOARD
100
IN
SENSE
GATE
C GATE
STAT
10
1
ON
MAX4271
MAX4272
MAX4273
0.01
t ON vs. CTIM (MAX4271/MAX4272)
CSPD
GND
CTIM
0.1
0.01
0.1
t ON vs. CTON (MAX4273)
1 10 100
1000
C SPD
C TIM
CAPACITANCE (nF)
* OPTIONAL (SEE TEXT)
Figure 8. Startup and Retry Timeout
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-
Figure 9. Operation with an External Capacitor
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
18
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