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.
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 = (I
LIMIT
)
2
R
ON
where I
LIMIT
is the preset current limit (2.1A max) and
R
ON
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.
MAX1922
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
1μF
100kΩ
OVERCURRENT
OUTPUT
OFF
ON
IN
OUT
OUTPUT
0.1μF*
MAX1922
OC
EN
GND
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.
*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.
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