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

operates in a continuous current-limit mode that limits

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 100k

Ω 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.

Triple USB Switch with Autoreset and

Fault Blanking

_______________________________________________________________________________________

9

SEL

ON_

OUT_ BEHAVIOR

GND

ON

GND

OFF

GND

OFF

ON

Table 1. On/Off Control