Micrel, Inc.

SM840021

March 2010

8

M9999-031810-B

Power Supply Filtering

The SM840021 provides separate power supply pins to

isolate any high switching noise from outputs to internal

core blocks. VDD and VDDA should be individually

connected to the power plane through vias. Bypass

capacitors should be used for each pin. Figure 2

illustrates how the power supply filter for 3.3 V and 2.5 V

is configured.

Figure 2.

Crystal Loading

Crystal Recommendations

This device requires a parallel resonance crystal.

Substituting a series resonance crystal will cause this

device to operate at the wrong frequency and violate the

ppm specifications.

To achieve low ppm error, the total capacitance of the

crystal must be considered in order to calculate

appropriate capacitive loading (CL).

Load Capacitance at each side: Trim Capacitance = Ct =

(2*CL-(Cb + Cd))

CL: Crystal load capacitance. Defined by manufacturer

Ct: External trim capacitors. (Trimmed CL Load

capacitance to get the right ppm)

Cb: Board capacitance (vias, traces, etc.)

Cd: Internal capacitance of the device (lead frame, bond

wires, pin, etc.)

Equivalent Series

Resistance (ESR)

Max.

Cut

Load

Cap.

Shunt

Cap.

Max.

Drive

Max.

70Ω

AT

7pF

0.1mW

Crystal Input Interface