LMH2190 Datasheet, PDF (14/32 Pages) National Semiconductor (TI) – Quad Channel 27 MHz Clock Tree Driver with I2C Interface

LMH2190
SNAS473H – JUNE 2009 – REVISED MAY 2013
V_BAT
C_BAT
1 PF
V_{OUT B1}
C_OUT
2.2 PF
V_BAT
A1
1.8V LDO, 10 mA
A3 CLK1
C_CLK
10 nF
C_IN
V_DD
470 pF _{SCLK_IN C1}
CLOCK
GND ^EN
SCLK_REQ C2
CLOCK
TREE
DRIVER
CPU/
BASE
BAND
R_1,2
5.1 k:
CONTROL
LOGIC
ENABLE A2
SCL D2
I²C
SDA D1
CLOCK
REQUEST
LMH2190 _B2
A4 CLK2
D4 CLK3
D3 CLK4
B3 CLK_REQ1
B4 ^CLK_REQ2
C4 ^CLK_REQ3
C3 ^CLK_REQ4
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PERIPHERAL
PERIPHERAL
PERIPHERAL
PERIPHERAL
V_SS
Figure 19. Typical LMH2190 Setup
PHASE NOISE
An important specification for oscillators and clock buffers is phase noise. It determines the timing and thus
accuracy of various peripheral devices in a cell phone such as Bluetooth, WLAN and DVB-H.
Phase noise is expressed in the frequency domain and is usually specified at a number of offset frequencies
from the carrier frequency. The phase noise of the oscillator and the LMH2190 together determine the phase
noise of the clock that is distributed to the peripheral devices. Therefore an additive phase noise is specified for
the LMH2190 rather than its total output phase noise since that depends on the TCXO connected to the
LMH2190.
Knowing the TCXO phase noise and the additive phase noise of the LMH2190, the total phase noise to the
peripheral can be calculated:
PN_TCXO
add.PN_LMH2190
PN = 10 LOG 10 ¹⁰ + 10
10
Where, PN is the total phase noise at the output of the LMH2190, PN_TCXO is the TCXO’s phase noise and
add.PN_LMH2190 is the additive phase noise of the LMH2190, all in dBc/Hz.
CLOCK TREE DRIVER
The clock tree driver consists of one input that drives 4 outputs (Figure 20). It is supplied by the highly accurate
1.8V LDO. In default configuration the outputs are switched on when the clock request inputs are high. The input
as well as the output can be configured in several ways though I²C programming.
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