Datasheet

AD9553
Rev. A | Page 23 of 44
Loop Filter
The charge pump in the PFD delivers current to the loop filter
(see Figure 30). The components primarily responsible for the
bandwidth of the loop filter are external and connect between
Pin 16 and Pin 17.
The internal portion of the loop filter has two configurations: one
is for low loop bandwidth applications (~170 Hz) and the other is
for medium (~20 kHz)/high (~75 kHz) bandwidth applications.
The low loop bandwidth condition applies when the feedback
divider value (N) is 214 (16,384) or greater. Otherwise, the
medium/high loop bandwidth configuration is in effect. The
feedback divider value depends on the configuration of the Ax
and Yx pins per Table 16.
08565-331
TO
VCO
3kΩ
FILTER
53pF
C1
R
AD9553
C2
375Ω
400kΩ
BUFFER
170pF
CONTROL
LOGIC
SWITCHES CHANGE
STATE FOR N ≥ 16384
LDO
1716
FROM
CHARGE
PUMP
Figure 30. External Loop Filter
The bandwidth of the loop filter primarily depends on three
external components (R, C1, and C2). There are two sets of recom-
mended values for these components corresponding to the low and
medium/high loop bandwidth configurations (see Table 17).
Table 17. External Loop Filter Components
A3 to A0 Pins R C1 C2
Loop
Bandwidth
0001 to 1100, and 1111 6.8 kΩ 47 nF 1 µF 0.17 kHz
1110
1
12 kΩ 51 pF 220 nF 20 kHz
1101 and 1110
2
12 kΩ 51 pF 220 nF 75 kHz
1
The 20 kHz loop bandwidth case only applies when the A3 pin to A0 pin =
1110 and the Y5 pin to Y0 pin = 111111.
2
The 75 kHz loop bandwidth case only applies when the A3 pin to A0 pin =
1101 and the Y5 pin to Y0 pin = 101101 through 110010, or when the A3 pin
to A0 pin = 1110 and the Y5 pin to Y0 pin = 110011.
To achieve the best jitter performance in applications requiring a
loop bandwidth of less than 1 kHz, C1 and C2 must have an
insulation resistance of at least 500 ΩF.
PLL Locked Indicator
The PLL provides a status indicator that appears at Pin 20
(LOCKED). When the PLL acquires phase lock, the LOCKED
pin switches to a Logic 1 state. When the PLL loses lock,
however, the LOCKED pin returns to a Logic 0 state.
Alternatively, the LOCKED pin serves as a test port allowing the
user to monitor one-of-four internal clocks. Register 0x17[3:1]
controls the test port as shown in Table 18.
Table 18. LOCKED Pin Output Control
Register 0x17[3:1] LOCKED Pin Output
0XX PLL locked indication (default)
100 Crystal oscillator clock signal
101 PFD pump-up clock divided-by-2
110 PFD reference input clock divided-by-2
111 PLL feedback to PFD clock divided-by-2
Output Dividers
The output divider section consists of three dividers: P
0
, P
1
, and P
2
.
The P
0
divider (or VCO frequency prescaler) accepts the VCO
frequency and reduces it by a factor of 5 to 11 (selectable). This
brings the frequency down to a range between 305 MHz and
810 MHz.
The output of the P
0
divider independently drives the P
1
divider
and the P
2
divider. The P
1
divider establishes the frequency at
OUT1 and the P
2
divider establishes the frequency at OUT2.
The P
1
and P
2
dividers are each programmable over a range of
1 to 1023, which results in a frequency at OUT1 or OUT2 that
is an integer submultiple of the frequency at the output of the P
0
divider.
Output Driver Configuration
The user has complete control over all configurable parameters
of the OUT1 and OUT2 drivers via the OUT1 and OUT2 driver
control registers (Register 0x32 and Register 0x34, respectively,
as shown in Figure 31). To alter the parameters from their default
values, the user must use the SPI port to program the driver
control registers as desired.
The OUT1 and OUT2 drivers are configurable in terms of the
following parameters:
• Logic family (via mode control)
• Pin function (via mode control but only applies to the
CMOS family)
• Polarity (only applies to the CMOS family)
• Drive current
• Power-down
Output Driver Mode Control
Three mode control bits establish the logic family and pin function
of the output drivers. The three bits originate either from Bits[5:3]
of Register 0x32 and Register 0x34 or from the decode logic
associated with the OM2 to OM0 pins as shown in Figure 31.
Note that Bit 0 of Register 0x32 and Register 0x34 determines
the source of the three mode control bits for the associated
output driver. Specifically, when Bit 0 of the register is Logic 0
(default), the source of the mode control bits for the associated
driver is the OM2 to OM0 pin decoder. When Bit 0 is Logic 1, the
source of the mode control bits is from Bits[5:3] of Register 0x32
and Register 0x34.