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

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CONDITION

MAX1940 BEHAVIOR

Output Short Circuit

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.

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

blanking timeout period.

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 2.5

Ω in the Typical Operating Characteristics.)

Thermal Fault

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