74HCT9046A Datasheet, PDF (12/43 Pages) NXP Semiconductors – PLL with bandgap controlled VCO

NXP Semiconductors
74HCT9046A
PLL with band gap controlled VCO
2.75
VCO_IN
2.50
(1)
(1)
(2)
2.75
VCO_IN
2.50
2.25
−25
0
25
phase error (ns)
001aak444
2.25
−25
0
25
phase error (ns)
001aak445
(1) Due to parasitic capacitance on PC2_OUT.
(2) Backlash time (dead zone).
a. Response with traditional voltage-switch
charge-pump PC2_OUT (74HCT4046A).
b. Response with current switch charge-pump
PC2_OUT as applied in the 74HCT9046A.
Fig 11. The response of a locked-loop in the vicinity of the zero crossing of the phase error
The design of the low-pass filter is somewhat different when using current sources. The
external resistor R3 is no longer present when using PC2 as phase comparator.
The current source is set by R_bias. A simple capacitor behaves as an ideal integrator now,
because the capacitor is charged by a constant current. The transfer function of the
voltage switch charge pump may be used. In fact it is even more valid, because the
transfer function is no longer restricted for small changes only. Further the current is
independent from both the supply voltage and the voltage across the filter. For one that is
familiar with the low-pass filter design of the 74HCT4046A a relation may show how R_bias
relates with a fictive series resistance, called R3'.
This relation can be derived by assuming first that a voltage controlled switch PC2 of the
74HCT4046A is connected to the filter capacitance C2 via this fictive R3' (see Figure 8b).
Then during the PC2 output pulse the charge current equals:
I_cp = ^V-----^C---^C-----^–_R----_3V---'--^C---²---⁽--⁰--⁾
With the initial voltage V_C2(0) at: 0.5V_CC = 2.5 V,
I_cp
= -2---.-5--
R3'
As shown before the charge current of the current switch of the 74HCT9046A is:
I_cp = 17 × -_R--²-_b--^.-_i-⁵-_a--_s
Hence:
R3‘ = -^R---₁^b---₇ⁱ--^a--^s(Ω)
74HCT9046A_6
Product data sheet
Rev. 06 — 15 September 2009
© NXP B.V. 2009. All rights reserved.
12 of 43