MAX1564_10 Datasheet, PDF (9/11 Pages) Maxim Integrated Products – Triple 1.2A USB Switch in 4mm x 4mm Thin QFN

Triple 1.2A USB Switch in 4mm x 4mm
Thin QFN
loads with additional bypass capacitance and/or large
startup currents while protecting the upstream power
source. No fault is reported if the output voltage rises
nominal within the 20ms blanking period.
Applications Information
Setting the Current Limit
The current limit for the MAX1564 is user programma-
ble using the SETI input. Connect a resistor from SETI
to GND (R1) to set the current limit. The value for R1 is
calculated as:
I_LIMIT = 1.37A x 26.1kΩ / R1
R1 must be between 26kΩ and 60kΩ.
Input Capacitor
To limit the input voltage drop during momentary output
load transients, connect a capacitor from IN_ to
ground. A 0.1µF ceramic capacitor is required for local
decoupling; however, higher capacitor values further
reduce the voltage drop at the input. When driving
inductive loads, a larger capacitance prevents voltage
spikes from exceeding the MAX1564’s absolute maxi-
mum ratings.
Output Capacitor
A capacitor as large as 2000µF may be used on the
output to smooth out transients and/or increase rise/fall
times. Larger output capacitance may be used, but the
resulting output charge time during startup may exceed
the fault blanking period, resulting in a FLT_ flag.
Driving Inductive Loads
A wide variety of devices (mice, keyboards, cameras,
and printers) typically connect to the USB port with
cables, adding an inductive component to the load. This
inductance causes the output voltage at the USB port to
oscillate during a load step. The MAX1564 drives induc-
tive loads; however, care must be taken to avoid
exceeding the device’s absolute maximum ratings.
Usually, the load inductance is relatively small, and the
MAX1564’s input includes a substantial bulk capaci-
tance from an upstream regulator, as well as local
bypass capacitors, limiting overshoot. If severe ringing
occurs because of large load inductance, clamp the
MAX1564 outputs below +6V and above -0.3V.
Turn-On and Turn-Off Behavior
Slow turn-on and turn-off under normal operating condi-
tions minimizes loading transients on the upstream
power source. Rapid turn-off under fault conditions
(thermal, short circuit, and UVLO) is done for maximum
safety.
Table 1. SEL/ON_ Inputs
SEL
High
High
Low
Low
ON_
High
Low
High
Low
OUT_ STATE
Enabled
Disabled
Disabled
Enabled
SEL sets the active polarity of the logic inputs of the
MAX1564. Connect ON_ to the same voltage as SEL to
enable the respective OUT_ switch. Connect ON_ to
the opposite voltage as SEL to disable the respective
output (see Table 1). The output of a disabled switch
enters a high-impedance state.
Layout and Thermal Dissipation
Keep all input/output traces as short as possible to
reduce the effect of undesirable parasitic inductance
and optimize the switch response time to output short-
circuit conditions. Place input and output capacitors no
more than 5mm from device leads. Connect IN_ and
OUT_ to the power bus with short traces. Wide power
bus planes at IN_ and OUT_ provide superior heat dissi-
pation as well. An active switch dissipates little power
with minimal change in package temperature. Calculate
the power dissipation for this condition as follows:
P = I_{OUT_}² x R_ON
At the normal operating current (I_{OUT_} = 0.5A) and the
maximum on-resistance of the switch (100mΩ), the
power dissipation is:
P = (0.5A)² x 0.100Ω = 25mW per switch
The worst-case power dissipation occurs when the out-
put current is just below the current-limit threshold with
an output voltage greater than 1V. In this case, the
power dissipated in each switch is the voltage drop
across the switch multiplied by the current limit:
P = I_LIM x (V_IN - V_OUT)
For a 5.5V input and 1V output, the maximum power
dissipation per switch is:
P = 1.54A x (5.5V - 1V) = 6.9W
Because the package power dissipation is 1349mW,
the MAX1564 die temperature may exceed the +160°C
thermal-shutdown threshold, in which case the switch
output shuts down until the junction temperature cools
by 10°C. In a continuous overload condition, this caus-
es a cyclical on/off situation. The duty cycle and period
of this situation are strong functions of the ambient tem-
perature and the PC board layout (see the Thermal
Shutdown section).
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