MAX14544 Datasheet, PDF (8/10 Pages) Maxim Integrated Products – 28V-Capable, ILIM VBUS Accessory Switch

28V-Capable, I_LIM V_BUS Accessory Switch
Detailed Description
The MAX14544/MAX14545 are overcurrent detection
switches that prevent host devices from being damaged
due to faulty load conditions. The switch opens if current
exceeds the current limit for longer than the blanking
time or if a reverse-current condition is detected. The
switch also disconnects if VIN exceeds OVLO, VIN falls
below UVLO, thermal shutdown occurs, or EN is driven
low. Current limit can be set to either 200mA or 400mA.
When the switch is open, the output (OUT) can withstand
28V maximum.
When the switch is on and a load is connected to the
port, a blanking time of 10.5ms ensures that the inrush
current settles down. After this blanking time, if the volt-
age across the switch is greater than the FLAG asser-
tion voltage, the part is turned off and issues a FLAG
to the microprocessor. After the retry time elapses, the
MAX14544 switch is closed for the duration of the blank-
ing time to check if the load current is lower than the
limit. Cycle the power or EN input to turn the MAX14545
switch back on.
Current-Limit Threshold
The MAX14544/MAX14545 ISET logic input sets the
desired minimum current-limit threshold to 200mA or
400mA. Connect ISET low for a 200mA current limit and
high for a 400mA current limit. If 400mA is selected as
the current limit, the input supply current at shutdown
can be reduced by connecting ISET to EN. This is due
to the fact that the device consumes the lower supply
current in ISET = low mode than in ISET = high mode.
Reverse-Current Protection
The MAX14544/MAX14545 monitor the reverse current
(VOUT to VIN) from exceeding the maximum IDET value.
If the reverse-current detection threshold is exceeded,
the switch turns off and FLAG is asserted without waiting
for tBLANK to elapse. This feature prevents excessive
reverse currents from flowing through the device.
FLAG Indicator
The MAX14544/MAX14545 feature a fault output (FLAG).
Whenever VIN - VOUT > VFA, the MAX14544 asserts
FLAG low after the blanking time and keeps it low until
the condition is removed. During this time, the switch
cycles on and off in the autoretry mode. When the condi-
tion is removed, FLAG deasserts and the switch turns
on (Figure 1). The MAX14545 asserts FLAG low after the
blanking time and turns the switch off when FLAG asser-
tion voltage is exceeded (Figure 2).
FLAG is an open-drain output and requires an external
pullup resistor. During shutdown (EN is low), the pull-
down on FLAG output is released to limit power dissipa-
tion. FLAG goes low when one of the following occurs:
1) The temperature exceeds the thermal shutdown tem-
perature, +150NC (typ).
2) The device is in current limit for more than the fault
blanking period.
3) The switch is in autoretry.
4) The reverse current limit is exceeded.
5) The input is over the OVLO.
Autoretry (MAX14544)
When the FLAG assertion voltage is exceeded, the
tBLANK timer begins counting (Figure 1). The timer resets
if the condition disappears before tBLANK has elapsed.
A retry time delay (tRETRY) is started immediately after
tBLANK has elapsed and during that time the switch is
turned off and FLAG is asserted. At the end of tRETRY,
the device is turned on again. If the fault still exists, the
cycle is repeated. If the fault is removed, the device
stays on and FLAG deasserts.
The autoretry feature saves system power in the case
of a continuous overcurrent or short-circuit condition.
During tBLANK, when the switch is on, the supply current
is at the current limit. During tRETRY, when the switch
is off, the current through the switch is zero. Instead
of observing the full load current, the device sees the
equivalent load current times duty cycle or:
ISUPPLY = ILOAD x tBLANK /(tBLANK + tRETRY)
With a typical tBLANK = 10.5ms and typical tRETRY =
661.5ms, the duty cycle is 1.6%, resulting in a 98.4%
power savings over the switch being on the entire time.
Latchoff (MAX14545)
When the FLAG assertion voltage is exceeded, the
tBLANK timer begins counting. The device turns off after
the blanking time. The timer resets if the condition is
removed before the end of the blanking time. Reset the
device by toggling EN (Figure 2).
Fault Blanking
The MAX14544/MAX14545 feature a 10.5ms (typ) fault
blanking. Fault blanking allows current-limit faults,
including momentary short-circuit faults that occur when
plugging a capacitive load, and also ensures that no
fault is issued during power-up. When a load transient
causes the device to enter current limit, an internal coun-
ter starts. If the load-transient fault persists beyond the
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