74hct9046a NXP Semiconductors, 74hct9046a Datasheet - Page 9

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

Philips Semiconductors
When the frequencies of SIG_IN and COMP_IN are equal
but the phase of SIG_IN leads that of COMP_IN, the up
output driver at PC2_OUT is held ‘ON’ for a time
corresponding to the phase difference (
phase of SIG_IN lags that of COMP_IN, the down or sink
driver is held ‘ON’.
When the frequency of SIG_IN is higher than that of
COMP_IN, the source output driver is held ‘ON’ for most of
the input signal cycle time and for the remainder of the
cycle time both drivers are ‘OFF’ (3-state). If the SIG_IN
frequency is lower than the COMP_IN frequency, then it is
the sink driver that is held ‘ON’ for most of the cycle.
Subsequently the voltage at the capacitor (C2) of the
low-pass filter connected to PC2_OUT varies until the
signal and comparator inputs are equal in both phase and
frequency. At this stable point the voltage on C2 remains
constant as the PC2 output is in 3-state and the VCO input
at pin 9 is a high impedance. Also in this condition the
signal at the phase comparator pulse output (PCP_OUT)
has a minimum output pulse width equal to the overlap
time, so can be used for indicating a locked condition.
2003 Oct 30
handbook, full pagewidth
PLL with band gap controlled VCO
I P
I P
0
2
Two kinds of transfer functions may be regarded:
a. The current transfer:
pump current
-------
2
I
P
PC_IN
0
PC_IN
2
PC_IN
V DEM_OUT(AV)
Fig.9 Phase comparator 2.
). When the
0.5V CC
V CC
0
2
b.The voltage transfer; this transfer can be
observed at PC2_OUT by connecting a resistor
(R = 10 k ) between PC2_OUT and 0.5V
V
DEM_OUT
9
Thus for PC2 no phase difference exists between SIG_IN
and COMP_IN over the full frequency range of the VCO.
Moreover, the power dissipation due to the low-pass filter
is reduced because both output drivers are OFF for most
of the signal input cycle. It should be noted that the PLL
lock range for this type of phase comparator is equal to the
capture range and is independent of the low-pass filter.
With no signal present at SIG_IN the VCO adjust, via PC2,
to its lowest frequency.
By using current sources as charge pump output on PC2,
the dead zone or backlash time could be reduced to zero.
Also, the pulse widening due to the parasitic output
capacitance plays no role here. This enables a linear
transfer function, even in the vicinity of the zero crossing.
The differences between a voltage switch charge pump
and a current switch charge pump are shown in Fig.11.
=
V
PC2_OUT
0
=
PC_IN
------ -
4
5
PC_IN
MSB306
2
CC
.
I P
;
PC_IN =
R
74HCT9046A
Product specification
SIG_IN
COMP_IN