SNAS668E August 2015 – September 2024 LMK03328
PRODUCTION DATA
Each PLL supports programmable loop bandwidth from 200 Hz to 1 MHz. The loop filter components–R2, C1, R3, and C3–can be configured by programming R67, R68, R69, and R70, respectively, for PLL1 and R82, R83, R84, and R85, respectively, for PLL2. C2 for each PLL is an external component that is added on the LF1 or LF2 pins. When PLL1 and/or PLL2 are configured in the fractional mode, R69.0 and/or R84.0 must be set to 1, respectively, and R118[2-0] and/or R132[2-0] must each be set to 0x7, respectively. When PLL1 and/or PLL2 are configured in the integer mode, R69.0 and/or R84.0 must be set to 0, respectively, and R118[2-0] and/or R132[2-0] must each be set to 0x3 for second-order (NOTE: R69 and R84 must each be set to 0x0), or 0x7 for third-order, respectively. To set the PLL1 and/or PLL2' loop bandwidth to 200 Hz, set R120.0 to 1 and/or R134.0 to 0. Figure 8-18 shows the loop filter structure of either PLL.
Setting the PLL to the best possible bandwidth is important to minimize output jitter. A high bandwidth (≥ 100 kHz) provides the best input signal tracking and is therefore desired with a clean input reference (clock generator mode). A low bandwidth (≤ 1 kHz) is desired if the input signal quality is unknown (jitter cleaner mode). TI provides the WEBENCH Clock Architect for easy selection of the right loop filter components.