LMH2190 Datasheet, PDF (11/24 Pages) National Semiconductor (TI) – Quad Channel 27 MHz Clock Tree Driver with I2C Interface

Application Information
The LMH2190 is a quad channel configurable clock distribu-
tion device which supplies a digital system clock to peripher-
als in mobile handsets or other applications. Examples of
peripherals are Bluetooth, Wireless LAN, and/or Digital Video
Broadcast-H (DVB-H).
The LMH2190 provides a solution to clocking issues such as
limited drive capability, frequency pulling and crosstalk. The
drive capability of a TCXO can be insufficient when traces are
long and/ or multiple peripherals are connected to one TCXO.
The LMH2190’s clock outputs can be configured indepen-
dently to drive capacitive loads up to 50 pF per channel. The
buffer function of the LMH2190 prevents frequency pulling of
the TCXO. Frequency pulling can occur when the TCXO ob-
serves varying loads. A peripheral device that shuts down can
cause this load variation for instance. Crosstalk between pe-
ripheral devices is minimal since each peripheral has its own
LMH2190 digital clock output. Also isolation from peripheral
to TCXO is ensured by use of the LMH2190.
Adding a component in the clock path inherently means
adding noise. The LMH2190 though has excellent phase
noise specifications in order to minimize degradation of the
clock quality. A typical LMH2190 application is depicted in
Figure 5.
The LMH2190 clock tree driver can be divided into 4 blocks:
• Clock tree driver
The clock tree driver provides a clean clock to a maximum
of 4 separately connected peripheral devices.
• Clock request logic
Independent clock request inputs allow the peripheral to
control when the particular clock should be enabled.
Further, the clock request inputs control the source clock
request (SCLK_REQ) and enabling of the LDO.
• Low Dropout regulator (LDO)
The LDO provides a low noise, high PSRR supply voltage
that enables low phase noise on the clock outputs, and low
quiescent current for portable applications. It can also be
used to provide a low noise supply to the TCXO eliminating
the need for a separate LDO.
• I²C Control logic
An I²C control port enables re-configuration of settings of
many features of the device in order to optimize the device
performance based on the application. For these settings
see Tables 2, 3, 4, 5, 6 in section I²C Registers.
All the blocks can be switched into a low power-consumption
mode to save energy. This functionality is controlled via the
ENABLE pin.
The following sections provide an explanation on phase noise
and a detailed description of each block.
FIGURE 5. Typical LMH2190 Setup
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