LMH2190 Datasheet, PDF (13/32 Pages) National Semiconductor (TI) – Quad Channel 27 MHz Clock Tree Driver with I2C Interface

LMH2190
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APPLICATION INFORMATION
SNAS473H – JUNE 2009 – REVISED MAY 2013
The LMH2190 is a quad channel configurable clock distribution device which supplies a digital system clock to
peripherals 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 independently 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 observes varying loads. A peripheral device that shuts down can
cause this load variation for instance. Crosstalk between peripheral devices is minimal since each peripheral has
its own LMH2190 digital clock output. Also isolation from peripheral to TCXO is specified 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 19.
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 Table 2, Table 3, Table 4,
Table 5, and Table 6 in 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.
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