MAX1922 データシートの表示（PDF） - Maxim Integrated

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MAX1922

1A Current-Limited Switch for 2 USB Ports

Maxim Integrated 

1A Current-Limited Switch for 2 USB Ports

outputs. Input voltage glitches less than 150mV do not

cause a spurious OC output. Load-transient faults less

than 10ms (typ) will not cause an OC output assertion.

Only current-limit faults are blanked. Die overtempera-

ture faults and input voltage droops below the UVLO

threshold will cause an immediate OC output.

Applications Information

Input Capacitor

To limit the input voltage drop during momentary output

short-circuit conditions, connect a capacitor from IN to

GND. A 1µF ceramic capacitor is adequate for most

applications; however, higher capacitor values further

reduce the voltage drop at the input (Figure 2).

INPUT

IN

OUT

1μF

OVERCURRENT

OUTPUT

100kΩ

MAX1922

OC

OFF

ON

EN

GND

OUTPUT

0.1μF*

*USB SPECIFICATIONS REQUIRE

A HIGHER VALUE CAPACITOR.

Figure 2. Typical Application Circuit

Output Capacitor

Connect a 0.1µF capacitor from OUT to GND. This

capacitor helps to prevent inductive parasitics from

pulling OUT negative during turn-off.

Layout and Thermal Dissipation

Important: Optimize the switch response time to output

short-circuit conditions by keeping all traces as short

as possible to reduce the effect of undesirable parasitic

inductance. Place input and output capacitors as close

as possible to the device (no more than 5mm away). All

IN and OUT pins must be connected with short traces

to the power bus. Wide power-bus planes provide

superior heat dissipation through the MAX1922’s IN

and OUT pins. Under normal operating conditions, the

package can dissipate and channel heat away.

Calculate the maximum power dissipation as follows:

P = (ILIMIT)2  RON

where ILIMIT is the preset current limit (2.1A max) and

RON is the on-resistance of the switch (125mΩ max).

When the output is short circuited, foldback-current lim-

iting activates and the voltage drop across the switch

equals the input supply voltage. The power dissipated

across the switch increases, as does the die tempera-

ture. If the fault condition is not removed, the thermal-

overload protection circuitry activates (see Thermal

Shutdown section). Wide power-bus planes connected

to IN and OUT and a ground plane in contact with the

device help dissipate additional heat.

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

during a load step. The MAX1922 is capable of driving

inductive loads, but avoid exceeding the device’s

absolute maximum ratings. Usually the load inductance

is relatively small, and the MAX1922 input includes a

substantial bulk capacitance from an upstream regula-

tor as well as local bypass capacitors, limiting over-

shoot. If severe ringing occurs due to large load

inductance, clamp the MAX1922 output below 6V and

above -0.3V.

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