between IN and AUX. However, a fault does not trigger

since the fault blanking-time is longer than the supply

switchover time.

Applications Information

Input Power Source

AUX provides the power for the MAX1838 internal con-

trol circuitry. Limit the input-voltage slew rate to less

than 0.2V/µs to prevent erroneous FAULT indications.

Input Capacitor

To limit the input voltage drop during momentary output

short-circuit conditions, connect a capacitor from both

AUX and IN to ground. A 0.1µF ceramic capacitor is

required for local decoupling; however, higher capaci-

tor values will further reduce the voltage drop at the

input (Figure 2). When driving inductive loads, a larger

capacitance prevents voltage spikes from exceeding

the MAX1838’s absolute maximum ratings.

Output Capacitor

An output capacitor helps prevent parasitic inductance

from pulling OUT negative during turn-off. When start-

ing up into very large capacitive loads, the switch puls-

voltage rises above 1V, then the capacitor continues to

charge at the full 0.9A current limit. There is no limit to

the size of output capacitor, but to prevent a startup

fault assertion, the capacitor must charge up within the

fault-blanking delay period. Typically starting up into a

330µF or smaller capacitor does not trigger a fault out-

put. In addition to bulk capacitance, small value (0.1µF

or greater) ceramic capacitors improve the output’s

resilience to electrostatic discharge (ESD).

Driving Inductive Loads

A wide variety of devices (mice, keyboards, cameras,

and printers) can load the USB port. These devices

commonly connect to the port with cables, which can

add an inductive component to the load. This induc-

tance causes the output voltage at the USB port to ring

Dual USB Switch with Fault Blanking

and Autoreset

10

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CONDITION

MAX1838 BEHAVIOR

If a short circuit is present

• If a short circuit is detected at the output, the channel turns off, the blanking timer

begins, and

FAULT stays high.

the next ramped current pulse soft-starts the output. The

FAULT flag stays high.

• If the short circuit persists after the fault-blanking period, FAULT goes low, autoreset

mode is enabled, and the output sources 25mA. If the voltage on that output is above

0.5V for 20ms, the channel resets, the output turns on, and

FAULT goes high (see

Short-Circuit Response in the Typical Operating Characteristics.)

If an overload current is present

or the 20ms continuous current-limit timeout period is reached.

• If overcurrent is still present at 20ms, FAULT goes low, autoreset mode is enabled, and

the output sources 25mA. If the voltage on that output is above 0.5V for 20ms, the

channel resets, the output turns on, and

FAULT goes high.

If thermal fault condition is present

• FAULT immediately goes low (the blanking timer does not apply to thermal faults), and

the channel turns off. When the junction cools by 15

removed, autoreset mode is enabled, the output sources 25mA, and

FAULT remains

low. If the voltage on that output is above 0.5V for 20ms, the channel resets, the output

turns on, and

FAULT goes high.

Table 2. MAX1838 Current Limiting and Fault Behavior