Datasheet

CPout
Vtune
Charge
Pump
Fastlock
Control
FLout
R2pLF
R2_LF
C2_LF
C1_LF
R3_LF
R4_LF
LMX2541SQ2060E, LMX2541SQ2380E
LMX2541SQ2690E, LMX2541SQ3030E
LMX2541SQ3320E, LMX2541SQ3740E
www.ti.com SNOSB31I –JULY 2009–REVISED FEBRUARY 2013
CE Pin POWERDOWN Bit Device State
0 Powered Up
High
1 Powered Down
The device can be programmed in the powerdown state. However, the VCO frequency needs to be changed
when the device is powered up because the VCO calibration does not run in the powerdown state. Also, the
special programming for VCO_DIV = 4 or 5 has to be done when the part is powered up. In order for the CE pin
to properly power the device down when it is held low, the all registers in the device need to have been
programmed at least one time.
Fastlock
The LMX2541 includes the Fastlock feature that can be used to improve the lock times. When the frequency is
changed, a timeout counter is used to engage the fastlock for a programmable amount of time. During the time
that the device is in Fastlock, the FLout pin changes from high impedance to low, thus switching in the external
resistor R2pLF with R2_LF as well as changing the internal loop filter values for R3_LF and R4_LF.
The following table shows the charge pump gain, loop filter resistors, and FLout pin change between normal
operation and Fastlock.
Normal
Parameter Fastlock
Operation
Charge Pump Gain CPG FL_CPG
Loop Filter Resistor R3_LF R3_LF FL_R3_LF
Loop Filter Resistor R4_LF R4_LF FL_R4_LF
High
FLout Pin Low
Impedance
Once the loop filter values and charge pump gain are known for normal mode operation, they can be determined
for fastlock operation as well. In normal operation, one can not use the highest charge pump gain and still use
fastlock because there will be no larger current to switch in. If the resistors and the charge pump current are
done simultaneously, then the phase margin can be preserved while increasing the loop bandwidth by a factor of
K as shown in the following table:
Parameter Symbol Calculation
Charge pump gain in Fastlock FL_CPG Typically choose to be the largest value.
K =
Loop Bandwidth Multiplier K
sqrt (FL_CPG/CPG)
FL_R3_LF =
Internal Loop Filter Resistor FL_R3_LF
R3_LF / K
FL_R4_LF =
Internal Loop Filter Resistor FL_R4_LF
R4_LF / K
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Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E
LMX2541SQ3320E LMX2541SQ3740E