Datasheet

Application Notes
81
SLLS519H—January 2010 TUSB3410, TUSB3410I
13 Application Notes
13.1 Crystal Selection
The TUSB3410 requires a 12-MHz clock source to work properly. This clock source can be a crystal placed across
the X1 and X2 terminals. A parallel resonant crystal is recommended. Most parallel resonant crystals are specified
at a frequency with a load capacitance of 18 pF. This load can be realized by placing 33-pF capacitors from each end
of the crystal to ground. Together with the input capacitance of the TUSB3410 and stray board capacitance, this
provides close to two 36-pF capacitors in series to emulate the 18-pF load requirement. Note, that when using a
crystal, it takes about 2 ms after power up for a stable clock to be produced.
When using a clock oscillator, the signal applied to the X1/CLKI terminal must not exceed 1.8 V. In this configuration,
the X2 terminal is unconnected.
TUSB3410
X1/CLKI
33 pF
12 MHz
X2
33 pF
Figure 13−1. Crystal Selection
13.2 External Circuit Required for Reliable Bus Powered Suspend Operation
TI has found a potential problem with the action of the SUSPEND output terminal immediately after power on. In some
cases the SUSPEND terminal can power up asserted high. When used in a bus powered application this can cause
a problem because the VREGEN
input is usually connected to the SUSPEND output. This in turn causes the internal
1.8-V voltage regulator to shut down, which means an external crystal may not have time to begin oscillating, thus
the device will not initialize itself correctly.
TI has determined that using components R2 and D1 (rated to 25 mA) in the circuit shown below can be used as a
workaround. Note that R1 and C1 are required components for proper reset operation, unless the reset signal is
provided by another means.
Note that use of an external oscillator (1.8-V output) versus a crystal would avoid this situation. Self-powered
applications would probably not see this problem because the VREGEN
input would likely be tied low, enabling the
internal 1.8-V regulator at all times.
TUSB3410
SUSPEND
D1
VREGEN
RESET
R2
32 kΩ
C1
1 μF
3.3 V
R1
15 kΩ
Figure 13−2. External Circuit