CL = 6 + 6 + 4 = 14 pF 2 CSTRAY" />
參數(shù)資料
型號(hào)： LMK04011BISQX/NOPB
廠商： National Semiconductor
文件頁數(shù)： 43/65頁
文件大?。?/td> 0K
描述： IC CLOCK CONDITIONER W/PLL 48LLP
標(biāo)準(zhǔn)包裝： 2,500
系列： PowerWise®
類型： 時(shí)鐘調(diào)節(jié)器
PLL： 是
輸入： LVCMOS
輸出： 2VPECL，LVPECL
電路數(shù)： 1
比率 - 輸入:輸出： 2:5
差分 - 輸入:輸出： 是/是
頻率 - 最大： 1.57GHz
除法器/乘法器： 是/是
電源電壓： 3.15 V ~ 3.45 V
工作溫度： -40°C ~ 85°C
安裝類型： 表面貼裝
封裝/外殼： 48-WFQFN 裸露焊盤
供應(yīng)商設(shè)備封裝： 48-LLP（7x7）
包裝： 帶卷 (TR)
其它名稱： LMK04011BISQX
2
CL = 6 + 6 +
4
= 14 pF
2
CSTRAY
CL = CTUNE + CIN +
C
Po
u
t1
LMK040xx
OSCin
OSCin*
Copt
PLL1 Loop Filter
XTAL
CC1 = 2.2 nF
CC2 = 2.2 nF
R1 = 4.7k
R3 = 10k
Copt
1 nF
R2 = 4.7k
SMV1249-074LF
SNOSAZ8J – SEPTEMBER 2008 – REVISED SEPTEMBER 2011
Figure 16. Reference Design Circuit for Crystal Oscillator Option
Optional Crystal Oscillator Implementation (OSCin/OSCin*)
The LMK04000 family features supporting circuitry for a discretely implemented oscillator driving the OSCin port
pins. Figure 16 illustrates a reference design circuit for a crystal oscillator:
This circuit topology represents a parallel resonant mode oscillator design. When selecting a crystal for parallel
resonance, the total load capacitance, CL, must be specified. The load capacitance is the sum of the tuning
capacitance (CTUNE), the capacitance seen looking into the OSCin port (CIN), and stray capacitance due to PCB
parasitics (CSTRAY), and is given by:
CTUNE is provided by the varactor diode shown in Figure 16, Skyworks model SMV1249-074. A dual diode
package with common cathode and provides the variable capacitance for tuning. The single diode capacitance
ranges from approximately 31 pF at 0.3 V to 3.4 pF at 3 V. The capacitance range of the dual package (anode to
anode) is approximately 15.5 pF at 3 V to 1.7 pF at 0.3 V. The desired value of VTUNE applied to the diode should
be VCC/2, or 1.65 V for VCC = 3.3 V. The typical performance curve from the data sheet for the SMV1249-074
indicates that the capacitance at this voltage is approximately 6 pF (12 pF/2).
The nominal input capacitance (CIN) of the LMK04000 family OSCin pins is 6 pF. The stray capacitance (CSTRAY)
of the PCB should be minimized by arranging the oscillator circuit layout to achieve trace lengths as short as
possible and as narrow as possible trace width (50
Ω characteristic impedance is not required). As an example,
assume that CSTRAY is 4 pF. The total load capacitance is nominally:
Consequently the load capacitance specification for the crystal in this case should be nominally 14 pF.
48
Copyright 2008–2011, Texas Instruments Incorporated
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