Dual USB Switch with Fault

Blanking and Autoreset

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Table 1. MAX1823 Current Limiting and Fault Behavior

Fault blanking allows the MAX1823 to handle USB loads

that may not be fully compliant with the USB specifica-

tions. USB loads with additional bypass capacitance

and/or large startup currents can be successfully pow-

ered even while protecting the upstream power source.

No fault is indicated if the switch is able to bring up the

load within the 20ms blanking period.

Applications Information

Input Power Source

IN, INA, and INB provide the power for all control and

charge-pump circuitry. All three IN_ pins must be con-

nected together externally. The input-voltage slew rate

should be less than 0.2V/µs to prevent erroneous

FAULT_ indications. This condition should not occur

under normal USB applications.

Input Capacitor

Connect a capacitor from IN_ to ground to limit the input

voltage drop during momentary output short-circuit con-

ditions. A 0.1µF ceramic capacitor is required for local

decoupling; higher capacitor values further reduce the

voltage drop at the input (see the Typical Application

Circuit (Figure 1)). When driving inductive loads, a larg-

er capacitance prevents voltage spikes from exceeding

the MAX1823’s absolute maximum ratings.

Output Capacitor

Place a 1µF or greater capacitor at each output for noise

immunity. When starting up into very large capacitive

until the output voltage rises above 1V, then the capacitor

continues to charge at the full 0.9A current limit. There is

no limit to the output capacitor size, but to prevent a start-

up 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 com-

monly connect to the port with cables, which can add an

inductive component to the load. This inductance causes

the output voltage at the USB port to ring during a load

step. The MAX1823 is capable of driving inductive loads,

but avoids exceeding the device’s absolute maximum rat-

ings. Usually the load inductance is relatively small, and

the MAX1823’s input includes a substantial bulk capaci-

tance from an upstream regulator as well as local bypass

capacitors, limiting overshoot. If severe ringing occurs

due to large load inductance, clamp the MAX1823’s out-

put below 6V and above -0.3V.

CONDITION

MAX1823 BEHAVIOR

Output short circuit

•

•

Removing the short circuit before the 15ms short-circuit blanking timeout period allows the

next ramped current pulse to soft-start the output. The FAULT_ flag stays high.

•

A short circuit exceeding 15ms to 20ms forces FAULT_ low at 20ms, enables autoreset

mode, and sources 25mA at the output.

•

An output voltage above 0.5V for 20ms resets the switch, turns on the output, and forces

FAULT_ high.

Output overload current

•

overload is removed, a thermal fault occurs, or the 20ms continuous current-limit timeout

period is reached.

•

An overcurrent condition still present at 20ms forces FAULT_ low, enables autoreset, and

sources 25mA at the output.

•

An output voltage above 0.5V for 20ms resets the switch, turns on the output, and forces

FAULT_ high.

Thermal fault

•

A junction temperature of +160°C immediately forces FAULT_ low (the blanking timer does

not apply to thermal faults) and turns off the switch. The junction cooling 15°C removes the

thermal fault condition, enables autoreset mode, and sources 25mA at the output. FAULT_

remains low while a thermal fault condition is present.

•

An output voltage above 0.5V for 20ms resets the switch, turns on the output, and forces

FAULT_ high.