SLLA549 July 2021 TCAN4550 , TCAN4550-Q1 , TCAN4551-Q1

4 Calculate the Load Capacitance

Figure 4-1 Crystal Load Capacitance

Without a TCAN455x device, crystal, and load capacitors mounted on the board, measure the parasitic capacitance between the crystal footprint pads. If series resistors are used, include those on the board when making the measurement. This becomes the PCB stray capacitance that will be used in calculations C_{PCB_Stray}.

The TCAN455x OSC1 Output Pin Capacitance Cout is typically 10 pF

The TCAN455x OSC2 Input Pin Capacitance Cin is typically 9 pF

Calculate the load capacitance presented to the crystal by the following formula:

Equation 3.

C_{L o a d} = (\frac{(C_{i n} + C_{L 2}) \times (C_{L 1} + C_{o u t})}{(C_{i n} + C_{L 2} + C_{L 1} + C_{o u t})}) + C_{P C B_S t r a y}

Where the OSC1 pin capacitance C_out is in parallel with the CL1 external load capacitance since they are both with respect to ground on the amplifier output side of the crystal and therefore added together.

Equation 4.

(C_{L 1} + C_{o u t})

Where the OSC2 pin capacitance C_in is in parallel with the CL2 external load capacitance since they are both with respect to ground on the amplifier input side of the crystal and therefore added together.

Equation 5.

(C_{i n} + C_{L 2})

Where the capacitance on both sides of the crystal are in series with each other relative to the crystal due to the common connection through the ground plane. Therefore they are calculated as capacitance in series.

Equation 6.

(\frac{(C_{i n} + C_{L 2}) \times (C_{L 1} + C_{o u t})}{(C_{i n} + C_{L 2} + C_{L 1} + C_{o u t})})

Where the parasitic capacitance of the PCB between the OSC1 and OSC2 pins is in parallel with the capacitance of the components and is therefore added to component capacitance.

Equation 7.

C_{P C B_S t r a y}

Adjust the value of the external capacitors CL1 and CL2 to achieve a total load capacitance C_Load of the recommended value specified in the crystal data sheet.