AK1590 Datasheet, PDF (14/36 Pages) Asahi Kasei Microsystems – 1GHz Delta-Sigma Fractional-N Frequency Synthesizer

[AK1590]
4. Fast Lockup Mode
Setting D[16] = {FASTEN} in <Address4> to “1” enables the Fast Lockup mode for AK1590.
Changing a frequency setting (The frequency changes at the rising edge of [LE], when <Address1> and <Address2> are
accessed.) or [PDN2] pin is turned from ”Low” to ”High” with {FASTEN}=1 enables the Fast Lockup 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 Lockup mode (Charge Pump 2) is enabled. After the timer period elapsed, the loop filter switch
turns OFF. The charge pump for normal operation (Charge Pump 1) is enabled. 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 can be adjusted with the register setting in 8 steps in normal operation (Charge Pump 1) and 8
steps in the Fast Lockup operation (Charge Pump 2).
The charge pump current for normal operation (Charge Pump 1) is determined by the setting in {CP1[2:0]}, which is a 3-bit
address of D[17:15] in <Address2>, and a value of the resistance connected to [BIAS] pin. The following formulas show
the relationship between the resistance value, the register setting and the current.
Charge Pump 1 minimum current (CP1_min) = 0.57 / Resistance connected to [BIAS] pin
Charge Pump 1 current = CP1_min × ({CP1[2:0]} + 1)
The charge pump current for the Fast Lockup mode operation (Charge Pump 2 current) is determined by the setting in
{CP2[2:0]}, which is a 3-bit address of D[15:13] in <Address4>, and a value of the resistance connected to [BIAS] pin. The
following formulas show the relationship between the resistance value, the register setting and the current.
Charge Pump 2 minimum current (CP2_min) = 5.7 / Resistance connected to [BIAS] pin
Charge Pump 2 current = CP2_min × ({CP2[2:0]} + 4)
The allowed range for the resistance (connected to [BIAS] pin) is from 22 to 33kΩ for both normal and Fast Lockup mode
operations. For details of current settings, see ”10. Register Functional Description”.
Operation
mode
Charge pump
Normal
Fast Lockup time specified
by the timer
Fast Lockup Mode
Normal
Charge Pump 1
Charge Pump 2
Charge Pump 1
Loop filter switch
OFF
ON
OFF
The frequency changes or [PDN2] pin is set from ”Low” to ”High”
during D[16] = {FSTEN} in < Address4 > is set to”1”.
Fig. 6 Timing Chart for Fast Lockup Mode
MS1478-E-01
14
2013/2