MAX1940
Applications Information
Input Power Supply and Capacitance
Connect all IN_ inputs together externally. IN_ powers
the internal control circuitry and charge pump for each
switch. Bypass IN_ to GND with a 0.1µF ceramic
capacitor. When driving inductive loads or operating
from inductive sources, which may occur when the
MAX1940 is powered by long leads or PC traces, larger
input bypass capacitance is required to prevent volt-
age spikes from exceeding the MAX1940’s absolute
maximum ratings during short-circuit events.
Output Capacitor
Bypass OUT_ to GND with a 1µF ceramic capacitor for
local decoupling. Additional bulk capacitance (up to
470µF) reduces output-voltage transients under
dynamic load conditions. Using output capacitors
greater than 470µF might assert FAULT_ if the current
limit cannot charge the output capacitor within the
20ms fault-blanking period. In addition to bulk capaci-
tance, small-value (0.1µF or greater) ceramic capaci-
tors improve the output’s resilience to electrostatic
discharge (ESD).
Driving Inductive Loads
A wide variety of devices (mice, keyboards, cameras,
and printers) typically connect to the USB port with
cables, which might add an inductive component to the
load. This inductance causes the output voltage at the
USB port to oscillate during a load step. The MAX1940
drives inductive loads, but avoid exceeding the
device’s absolute maximum ratings. Usually, the load
inductance is relatively small, and the MAX1940’s input
includes a substantial bulk capacitance from an
upstream regulator as well as local bypass capacitors,
limiting overshoot. If severe ringing occurs because of
large load inductance, clamp the MAX1940 outputs
below +6V and above -0.3V.
Turn-On and Turn-Off Behavior
The MAX1940’s slow turn-on and turn-off minimizes
load transients on the upstream power source. Under
fault conditions, the outputs of the MAX1940 turn off
rapidly to provide maximum safety for the upstream
power source and downstream devices. Internal blocks
shut down to minimize supply current when all three
channels are off.
Layout and Thermal Dissipation
Keep all traces as short as possible to reduce the
effect of undesirable parasitic inductance and optimize
the switch response time to output short-circuit condi-
tions. Place input and output capacitors no more than
5mm from device leads. Connect IN_ and OUT_ to the
Triple USB Switch with Autoreset and
Fault Blanking
10 ______________________________________________________________________________________
CONDITION MAX1940 BEHAVIOR
Output Short Circuit
(V
OUT_
< 1V)
If a short is detected at the output, the channel turns off, and the blanking timer begins. FAULT_ remains
high during the blanking timeout period.
If the short persists during the fault-blanking period, the output pulses at 0.35A
RMS
. If the short is
removed before the 18ms short-circuit blanking timeout period, the next ramped current pulse soft-starts
the output. FAULT_ remains high.
If the short circuit persists after the fault-blanking period, FAULT_ goes low, autoreset mode begins, and
the output sources 25mA.
If the output voltage rises above 0.5V for 20ms, the output turns on and FAULT_ goes high (see Short-
Circuit Response in the Typical Operating Characteristics.)
Output Overload Current
(V
OUT_
> 1V)
Output current regulates at I
LIM
and the blanking timer turns on. FAULT_ remains high during the
blanking timeout period.
Continuous current at I
LIM
persists until either the 20ms blanking period expires or a thermal fault occurs.
If overcurrent persists after 20ms, FAULT_ goes low, autoreset mode is enabled, and the output sources
25mA.
If the output voltage rises above 0.5V for 20ms, the output turns on and FAULT_ goes high (see Overload
Response into 2.5Ω in the Typical Operating Characteristics.)
Thermal Fault
(T
J
> +160°C)
A junction temperature of +160°C immediately asserts FAULT_ low (the blanking timeout period does not
apply for thermal faults) and turns off the switch. When the junction cools by 15°C, the thermal fault is
cleared and FAULT_ goes high. Note that if other fault conditions are present when a thermal fault clears,
those fault states take effect.
Table 2. Current-Limiting and Fault Behavior
