AK1542A Datasheet, PDF (13/37 Pages) Asahi Kasei Microsystems – 20 to 600MHz Integer-N Frequency Synthesizer

[AK1542A]
3. Fast Lock Up Mode
Setting D[16]={FASTEN} in <Address4> to 1 enables the Fast Lock Up mode for AK1542A.
Changing a frequency setting (The frequency is changed at the rising edge of [LE] when <Address1> and <Address2> are
accessed.) or [PDN2] pin is set from ”Low” to ”High” with {FASTEN}=1 enables the Fast Lock Up mode. The loop filter switch
turns ON during the timer period specified by the counter value in D[12:0] = {FAST[12:0]} in <Address4>, and the charge
pump for the Fast Lock Up mode (Charge Pump 2) set by D[9:6] = {CP2[3:0]} in <Address2> is enabled.
After the timer period elapsed, the loop filter switch turns OFF, the charge pump for normal operation (Charge Pump 1) set by
D[3:0] = {CP1[3:0]} in <Address2> is enabled and thus normal operation returns.
The register D[12:0] = {FAST[12:0]} in <Address4> is used to set the timer period for this mode. The following formula is used
to calculate the time period:
Phase detector frequency cycle × counter value set in {FAST[12:0]}
The charge pump current could be adjusted with 9 steps for both normal operation (Charge Pump 1) and the Fast Lock Up
operation (Charge Pump 2).
The absolute value of the charge pump current is determined by the Resistor value connected to the [BIAS] pin. The following
formula shows the relationship between the resistance value, the register setting and the electric current value.
Charge pump minimum current (Icp_min) [A] = 8.55 / Resistance connected to the [BIAS] pin (Ω)
Charge pump current [A] = Icp_min [A] × (CP1 or CP2 setting + 1)
The allowed range value for the resistance connected to the [BIAS] pin is from 22 to 33 [kΩ]. For details of current settings,
see ”Register Functional Description”.
Fast lock up time specified
by the timer
Operation mode
Normal
Fast Lock Up Mode
Normal
Charge pump
Charge Pump 1
Charge Pump 2
Charge Pump 1
Lop filter switch
OFF
ON
OFF
The frequency is changed or [PDN2] pin is set to ”High” when D[16]={FASTEN}
in <Address4> is set to”1”.
Fig. 6 Timing Chart for Fast Lock Up Mode
MS1399-E-00
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2012/3