LMX2313UEVAL National Semiconductor, LMX2313UEVAL Datasheet - Page 24

• Manufacturer Part Number: LMX2313UEVAL
• Manufacturer: National Semiconductor

Specifications of LMX2313UEVAL

• Lead Free Status / Rohs Status: Not Compliant

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1.0 Functional Description
1.9 Fastlock
The FL pin can be used as the Fastlock output. The FL pin
can also be programmed as constant low, constant high
(referenced to V
through the T register. When the device is configured in
Fastlock mode, the charge pump current can be increased
4x while maintaining loop stability by synchronously switch-
ing a parallel loop filter resistor to ground with the FL pin,
resulting in a ∼2x increase in loop bandwidth. The loop
bandwidth, the zero gain crossover point of the open loop
gain, is effectively shifted up in frequency by a factor of the
square root of 4 = 2 during Fastlock mode. For ω' = 2 ω, the
phase margin during Fastlock also will remain constant. The
user calculates the loop filter component values for the
normal steady state considerations. The device configura-
tion ensures that as long as a second resistor, equal to the
primary resistor value, is wired in appropriately, the loop will
lock faster without any additional stability considerations.
™
OUTPUT
CC
), or constant high impedance, selectable
(Continued)
24
The PLL can be configured to be in either the Fastlock mode
continuously or in the Fastlock mode that uses a timeout
counter to switch it back to the normal mode. In the Fastlock
mode the charge pump current is set to 4 mA and the FL pin
is set low. If the user sets the PLL to be in the Fastlock mode
continuously he can send the R register with CPo_4X set low
(R[18] = 0) and sets TO_CNTR[11:0] to 1. The user can set
the PLL to normal mode (1 mA mode and set the FL pin to
TRI-STATE mode) by programming TO_CNTR[11:0] to 0. If
the user elects to use the timeout counter, he can program
the timeout counter from 4 to 4095. The timeout counter will
count down the programmed number of phase detector ref-
erence cycles. After the programmed number of phase de-
tector reference cycles is reached, it will automatically set
the charge pump current to the 1 mA mode and set the FL
pin to TRI-STATE mode. A complete programming descrip-
tion is in Section 3.4.2.
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