STCD2400 Datasheet, PDF (12/39 Pages) STMicroelectronics – Multichannel clock distribution circuit

Device operation
3
Device operation
STCD22x0, STCD23x0, STCD24x0
3.1
Note:
Operation
The STCD22x0, STCD23x0 and STCD24x0 are 2, 3 or 4 buffered clock distribution circuits.
They accept the clock (either square wave or sine wave) input from an external clock source
and send 2, 3 or 4 buffered rail-to-rail (0 V to VTCXO) square wave outputs to different
devices. A 1.8 V, high PSRR LDO (VTCXO) is also integrated in the STCD22x0, STCD23x0
and STCD24x0 which can be used as a voltage supply for the external master clock source
(such as a TCXO). This LDO stops the current increase through PMOS when the load
current reaches the limit value of the current-limit protection circuit. When the load current
falls below the limit values, the current limit is released.
Each of the STCD22x0, STCD23x0 and STCD24x0 clock outputs can be enabled
individually. If the device connected to the output is in standby, and does not require a clock,
the buffered output can be disabled to save power consumption. Once the buffered output is
disabled, it is pulled down to GND internally. If all the devices connected are in standby, the
STCD22x0, STCD23x0 and STCD24x0 are also put into standby mode (the internal LDO is
also shut down) for further power consumption savings. All of the output enable signals are
logic ORed with an open drain output (MC_REQ) to control the output of the source clock. If
the output clock is required by at least one device, the LDO wakes up and the MC_REQ
activates the clock source. The truth table for enable signals, the master clock request signal
and the VTCXO is given in Table 5. The truth table for enable signals, output clock signals
and the master clock is given in Table 6.
The STCD22x0, STCD23x0 and STCD24x0 have the master clock input detector integrated.
If the input master clock peak-to-peak voltage is below the minimum specified level, even if
the outputs are enabled, there are no clock outputs and STCD22x0, STCD23x0 and
STCD24x0 enter standby mode. Once the master clock peak-to-peak voltage level reaches
the minimum value, the output clocks are asserted if the enable pins are active.
In Table 5 and 6, the enable signals are active high and the MC_REQ is active low. These
enable signals can be active high or active low. The enable polarity is described in
Section 3.2: Enable polarity. Customers can select different polarity options for different
applications. Contact the STMicroelectronics local sales office for availability.
Table 5.
Truth table for clock enable (EN1-4), master clock request (MC_REQ) and
VTCXO
EN1
EN2
EN3
EN4
MC_REQ
VTCXO
0
0
0
0
1
GND
1
0
0
0
0
1.8 V
1
1
0
0
0
1.8 V
-
-
-
-
0
1.8 V
1
1
1
1
0
1.8 V
"0" means logic low which disables the clock output and "1" means logic high which enables
the clock output. This is an active high truth table. Refer to Section 3.2: Enable polarity for
the detailed enable active high/low options.
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