On/Off Control and
Undervoltage Lockout
SEL sets the active polarity of the logic inputs of the
MAX1940. Connect ON_ to the same voltage as SEL to
enable the respective OUT_ switch. Connect ON_ to
the opposite voltage as SEL to disable the respective
output (see Table 1). The output of a disabled switch
enters a high-impedance state.
The MAX1940 includes a UVLO circuit to prevent erro-
neous switch operation when the input voltage goes low
during startup and brownout conditions. Input voltages of
less than 3.4V inhibit operation of the device. FAULT_
asserts low during an undervoltage lockout condition.
Output Fault Protection and
Autoreset
The MAX1940 senses the switch output voltage and
selects continuous current limiting for V
OUT_
greater
than 1V, or short-circuit current limiting for V
OUT_
less
than 1V. When V
OUT_
is greater than 1V, the device
operates in a continuous current-limit mode that limits
output current to 0.9A. When V
OUT_
is less than 1V, the
device operates in short-circuit current-limit mode,
sourcing 1.2A pulses to the load. When either fault con-
dition persists for 20ms, the output turns off and its fault
flag is asserted. The output automatically restarts 20ms
after the short or overload is removed.
The MAX1940 detects short-circuit removal by sourcing
25mA from the output and monitoring the output volt-
age. When the voltage at the output exceeds 0.5V for
20ms, the fault flag resets, the output turns back on,
and the 25mA current source turns off. Active loads are
not expected to have measurable current when sup-
plied with less than 0.5V.
Thermal Shutdown
Independent thermal shutdown for each channel per-
mits normal operation of two switches to continue while
a third experiences a thermal fault. The switch turns off
and the FAULT_ output asserts low immediately when
the junction temperature exceeds +160°C. Thermal
shutdown does not utilize the 20ms fault-blanking time-
out period. The switch turns on again and FAULT_
returns high when the junction temperature cools by
+15°C. The switch cycles on and off if the overload
condition persists, resulting in a pulsed output that
reduces system power.
Fault Indicators
The MAX1940 provides an independent open-drain fault
output (FAULT_) for each switch. Connect FAULT_ to IN_
through a 100kΩ pullup resistor for most applications.
FAULT_ asserts low when any of the following conditions
occur:
• The input voltage is below the UVLO threshold.
• The switch junction temperature exceeds the +160°C
thermal-shutdown temperature limit.
• The switch is in current-limit or short-circuit current-
limit mode after the fault-blanking period (20ms)
expires.
The FAULT_ output deasserts after a 20ms delay once
the fault condition is removed. Ensure that the MAX1940
input bypass capacitance prevents glitches from trigger-
ing the FAULT_ outputs. Limit the input voltage slew rate
to 0.2V/µs to prevent erroneous FAULT_ indications.
To differentiate large capacitive loads from short cir-
cuits or sustained overloads, the MAX1940 has an
independent fault-blanking circuit for each switch.
When a load transient causes the device to enter cur-
rent limit, an internal counter monitors the duration of
the fault. For load faults exceeding the 20ms fault-
blanking time, the switch turns off, FAULT_ asserts low,
and the device enters autoreset mode (see the
Output
Fault Protection and Autoreset Mode
section). Only cur-
rent-limit and short-circuit faults are blanked. Thermal
overload faults and input voltage drops below the
UVLO threshold immediately turn the switch off and
assert FAULT_ low.
Fault blanking allows the MAX1940 to handle USB
loads that might not be fully compliant with USB specifi-
cations. The MAX1940 successfully powers USB loads
with additional bypass capacitance and/or large start-
up currents while protecting the upstream power
source. No fault is reported if the switch brings up the
load within the 20ms blanking period. See Table 2 for a
summary of current-limit and fault behavior.
MAX1940
Triple USB Switch with Autoreset and
Fault Blanking
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