Datasheet

CPout
Vtune
R2_LF
C2_LF
C1_LF
R3_LF
R4_LF
Charge
Pump
LMX2541SQ2060E, LMX2541SQ2380E
LMX2541SQ2690E, LMX2541SQ3030E
LMX2541SQ3320E, LMX2541SQ3740E
www.ti.com SNOSB31I –JULY 2009–REVISED FEBRUARY 2013
Word Name Function
This needs to be set correctly if the internal VCO is used for proper
OSC_FREQ
calibration.
This allows the oscillator frequency to be doubled. The R divider is
OSC2X
bypassed in this case.
Higher slew rates tend to yield the best fractional spurs and phase noise, so a square wave signal is best for
OSCin. Single 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.
PLL R Divider
The R divider divides the OSCin frequency down to the phase detector frequency. If the doubler is enabled, then
the R divider is bypassed.
PLL Phase Detector and Charge Pump
The phase detector compares the outputs of the R and N dividers and generates a correction current
corresponding to the phase error. This charge pump current is software programmable to 32 different levels. The
phase detector frequency, f
PD
, can be calculated as follows:
f
PD
= f
OSCin
/ R (1)
PLL N Divider and Fractional Circuitry
The N divider in the LMX2541 includes fractional compensation 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:
N = PLL_N + PLL_NUM / PLL_DEN (2)
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 fractional spurs consistent, the
modulator is reset any time that the R0 register is programmed.
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 interface. The larger the values of these components, the stronger the attenuation of the
internal loop filter. The maximum attenuation can be achieved by setting the internal resistors and capacitors to
their maximum value and the minimum 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.
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:
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Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E
LMX2541SQ3320E LMX2541SQ3740E