特性 Features

? Digital Design Avoids Analog Compensation Errors

? Easily Cascadable for Higher Order Loops

? Useful Frequency Range

- K-Clock . . . . . . . . . . . . . . . . . . . . . . . . . .DC to 55MHz (Typ)

- I/D-Clock . . . . . . . . . . . . . . . . . . . . DC to 35MHz (Typ)

? Dynamically Variable Bandwidth

? Very Narrow Bandwidth Attainable

? Power-On Reset

? Output Capability

- Standard. . . . . . . . . . . . . . . . . . . . XORPDOUT, ECPDOUT

- Bus Driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I/DOUT

? Fanout (Over Temperature Range)

- Standard Outputs . . . . . . . . . . . . . . . . . . 10 LSTTL Loads

- Bus Driver Outputs . . . . . . . . . . . . . 15 LSTTL Loads

? Balanced Propagation Delay and Transition Times

? Significant Power Reduction Compared to LSTTL

Logic ICs

? ’HC297 Types

- Operation Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 to 6V

- High Noise ImmunityNIL = 30%, NIH = 30% of VCC at 5V

? CD74HCT297 Types

- Operation Voltage . . . . . . . . . . . . . . . . . . . . . . . . 4.5 to 5.5V

- Direct LSTTL Input Logic Compatibility

VIL = 0.8V (Max), VIH = 2V (Min)

- CMOS Input Compatibility II ≤ 1μA at VOL, VOH

描述 Description

The ’HC297 and CD74HCT297 are high-speed silicon gate

CMOS devices that are pin-compatible with low power Schottky

TTL (LSTTL).

These devices are designed to provide a simple, cost-effective

solution to high-accuracy, digital, phase-locked-loop applications.

They contain all the necessary circuits, with the

exception of the divide-by-N counter, to build first-order

phase-locked-loops.

Both EXCLUSIVE-OR (XORPD) and edge-controlled phase

detectors (ECPD) are provided for maximum flexibility. The

input signals for the EXCLUSIVE-OR phase detector must

have a 50% duty factor to obtain the maximum lock-range.

Proper partitioning of the loop function, with many of the building

blocks external to the package, makes it easy for the

designer to incorporate ripple cancellation (see Figure 2) or to

cascade to higher order phase-locked-loops.

The length of the up/down K-counter is digitally programmable

according to the K-counter function table. With A, B, C and D

all LOW, the K-counter is disabled. With A HIGH and B, C and

D LOW, the K-counter is only three stages long, which widens

the bandwidth or capture range and shortens the lock time of

the loop. When A, B, C and D are all programmed HIGH, the

K-counter becomes seventeen stages long, which narrows

the bandwidth or capture range and lengthens the lock time.

Real-time control of loop bandwidth by manipulating the A to

D inputs can maximize the overall performance of the digital

phase-locked-loop.

The ’HC297 and CD74HCT297 can perform the classic first

order phase-locked-loop function without using analog components.

The accuracy of the digital phase-locked-loop

(DPLL) is not affected by VCC and temperature variations but

depends solely on accuracies of the K-clock and loop propagation

delays.