LMX2541SQE2060E/NOPB National Semiconductor, LMX2541SQE2060E/NOPB Datasheet - Page 34

• Manufacturer Part Number: LMX2541SQE2060E/NOPB
• Description: IC PLL FREQ SYNTH W/VCO 36LLP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Clock/Frequency Synthesizer (RF)r

Specifications of LMX2541SQE2060E/NOPB

• Pll: Yes
• Input: Clock
• Output: Clock
• Number Of Circuits: 1
• Ratio - Input:output: 2:2
• Differential - Input:output: No/No
• Frequency - Max: 2.24GHz
• Divider/multiplier: Yes/No
• Voltage - Supply: 3.15 V ~ 3.45 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 36-LLP
• Frequency-max: 2.24GHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LMX2541SQE2060ETR

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1.0 Functional Description
The LMX2541 is a low power, high performance frequency
synthesizer system which includes a PLL, Partially Integrated
Loop Filter, VCO, VCO Divider, and Programmable Output
Buffer. There are three basic modes that the device can be
configured in: Full Chip Mode, External VCO Mode, and Di-
vider Only Mode. Full chip mode is intended to be used with
the internal VCO and PLL. There is also the option of External
VCO mode, which allows the user to connect their own ex-
ternal VCO. Finally, there is Divider only, which is just the
VCO divider and output buffer. The active blocks for these
modes are described below:
1.1 PLL REFERENCE OSCILLATOR INPUT PINS
There are three basic ways that the OSCin/OSCin* pins may
be configured as shown in the table below:
In addition to the way that the OSCin/OSCin* pins are driven,
there are also bits that effect the frequency that the chip uses.
The OSC_FREQ word needs to be programmed correctly, or
the VCO may have issues locking to the proper frequency,
since the VCO frequency calibration is based on this word.
Higher slew rates tend to yield the best fractional spurs and
phase noise, so a square wave signal is best for OSCin. Sin-
gle ended mode and differential mode have similar results if
a square wave is used to drive the OSCin pin. If using a sine
wave, higher frequencies tend to work better due to their
higher slew rates.
1.2 PLL R DIVIDER
The R divider divides the OSCin frequency down to the phase
detector frequency. If the doubler is enabled, then the R di-
vider is bypassed.
External
Differential
Divider
Mode
Chip
VCO
Only
Full
Crystal
Ended
Single
Mode
Word Name
OSC_FREQ
OSC2X
PLL
Yes
Yes
No
Device is used with a crystal
oscillator
Device is driven with a single-
ended source, such as a
TCXO.
Use this mode when driving
with a differential signal, such
as an LVDS signal.
Loop
Filter
Yes
No
No
Description
Available Blocks
This needs to be set correctly
if the internal VCO is used for
proper calibration.
This allows the oscillator
frequency to be doubled. The
R divider is bypassed in this
case.
VCO
Yes
No
No
Function
Divider
VCO
Yes
Yes
Yes
XO Bit
Output
Buffer
1
0
0
Yes
Yes
Yes
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1.3 PLL PHASE DETECTOR AND CHARGE PUMP
The phase detector compares the outputs of the R and N di-
viders and generates a correction current corresponding to
the phase error. This charge pump current is software pro-
grammable to 32 different levels. The phase detector fre-
quency, f
1.4 PLL N DIVIDER AND FRACTIONAL CIRCUITRY
The N divider in the LMX2541 includes fractional compensa-
tion and can achieve any fractional denominator (PLL_DEN)
from 1 to 4,194,303. The integer portion, PLL_N, is the whole
part of the N divider value and the fractional portion,
PLL_NUM / PLL_DEN, is the remaining fraction. PLL_N,
PLL_NUM, and PLL_DEN are software programmable. So in
general, the total N divider value, N, is determined by:
The order of the delta sigma modulator is programmable from
integer mode to fourth order. There are also several dithering
modes that are also programmable. In order to make the frac-
tional spurs consistent, the modulator is reset any time that
the R0 register is programmed.
1.5 PARTIALLY INTEGRATED LOOP FILTER
The LMX2541 integrates the third pole (formed by R3_LF and
C3_LF) and fourth pole (formed by R4_LF and C4_LF) of the
loop filter. The values for these integrated components can
be programmed independently through the MICROWIRE in-
terface. The larger the values of these components, the
stronger the attenuation of the internal loop filter. The maxi-
mum attenuation can be achieved by setting the internal
resistors and capacitors to their maximum value and the min-
imum attenuation can be attained by setting all of these to
their minimum setting. This partially integrated loop filter can
only be used in full chip mode.
1.6 LOW NOISE, FULLY INTEGRATED VCO
The LMX2541 includes a fully integrated VCO, including the
inductors. The VCO (Voltage Controlled Oscillator) takes the
voltage from the loop filter and converts this into a frequency.
The VCO frequency is related to the other frequencies and
divider values as follows:
In order to the reduce the VCO tuning gain and therefore im-
prove the VCO phase noise performance, the VCO frequency
range is divided into many different frequency bands. This
creates the need for frequency calibration in order to deter-
mine the correct frequency band given a desired output fre-
quency. The frequency calibration routine is activated any
time that the R0 register is programmed. It is important that
PD
, can be calculated as follows:
N = PLL_N + PLL_NUM / PLL_DEN
f
VCO
= f
PD
f
PD
× N = f
= f
OSCin
OSCin
/ R
× N / R
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