AN3271

Timing accuracy note

Doc ID 17933 Rev 2

9/29

3

Timing accuracy note

External capacitor-adjusted timings and environmental considerations

There are several external factors to be considered that may affect the accuracy of the

datasheet applies solely to the STM660x devices (i.e. it does not include an ideal timing

capacitor, external tolerances, temperature dependencies nor leakages).

The STM660x device is designed to meet strict requirements for the lowest possible current

consumption and to maintain the basic timing constant of 10 s / µF, therefore the constant

current used to charge the external timing capacitor is low, in the order of 100 nA. Any

external leakage due to poor quality timing capacitors or excessive humidity (especially if

the dew point is exceeded and moisture condenses on the PCB tracks) may cause a

significant leakage current which is deducted from the constant charging current that the

device provides. This results in a reduction of the charging current of the real external timing

by as great a distance as possible (see Figure 6 for recommended layout). Also, low-

leakage timing capacitors (ceramic or film capacitor) should be used.

Figure 6.

Recommended layout