74hct9046a NXP Semiconductors, 74hct9046a Datasheet - Page 11

• Manufacturer Part Number: 74hct9046a
• Description: Pll With Bandgap Controlled Vco
• Manufacturer: NXP Semiconductors

Philips Semiconductors
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
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
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 Fig.8b). Then during the PC2 output pulse the charge
current equals:
With the initial voltage V
0.5V
As shown before the charge current of the current switch
of the 74HCT9046A is:
Hence:
2003 Oct 30
R3'
I
I
b
P
P
PLL with band gap controlled VCO
relates with a fictive series resistance, called R3'.
=
=
CC
=
17
V
---------------------------------
= 2.5 V,
R
------ -
17
CC
b
R3'
2.5
------- -
–
R
V
b
C2 0
I
P
=
-------- -
R3'
2.5
C2(0)
at:
b
. A simple capacitor
11
Using this equivalent resistance R3' for the filter design the
voltage can now be expressed as a transfer function of
PC2; assuming ripple (f
Again this illustrates the supply voltage independent
behaviour of PC2.
LOOP FILTER COMPONENT SELECTION
Examples of PC2 combined with a passive filter are shown
in Figs 12 and 13. Figure 12 shows that PC2 with only a
C2 filter behaves as a high-gain filter. For stability the
damped version of Fig.13 with series resistance R4 is
preferred.
Practical design values for R
with R3' = 1.5 to 15 k for the filter design. Higher values
for R3' require lower values for the filter capacitance which
is very advantageous at low values the loop natural
frequency
K
PC2
=
------ - V r
4
5
n
.
r
= f
b
i
) is suppressed, as:
are between 25 and 250 k
74HCT9046A
Product specification