S35390A datasheet(41/51 Pages) SII

41 / 51 page

FOR AUTOMOTIVE 105°C OPERATION 2-WIRE REAL-TIME CLOCK

Rev.2.2_00

S-35390A H Series

Seiko Instruments Inc.

3. Adjustment of oscillation frequency

3. 1 Adjustment by setting Cg

Matching of the crystal oscillator with the nominal frequency must be performed with the parasitic capacitance on the

board included. Select a crystal oscillator and optimize the Cg value in accordance with the flowchart below.

START

END

YES

Set to center

of variable

capacitance*3

Select a crystal

oscillator*1

Variable

capacitance

Change Cg

Optimal

value*2

Frequency

Cg in

specification

Set Cg

Make fine adjustment

of frequency using

variable capacitance

Trimmer capacitor

Fixed capacitor

*1.

Request a crystal manufacturer for a matching evaluation between the IC and the crystal oscillator. The

recommended crystal characteristic value is CL value (load capacitance) = 6 pF. When using the IC and a crystal

oscillator in the environment exceeding Ta =

+85°C, it is recomme

nded to ensure that the oscillation allowance is at least seven times at normal temperature.

*2.

The Cg value must be selected on the actual PCB since it is affected by parasitic capacitance. Select the

external Cg value in a range of 0 pF to 9.1 pF.

*3.

Adjust the rotation angle of the variable capacitance so that the capacitance value is slightly smaller than the

center, and confirm the oscillation frequency and the center value of the variable capacitance. This is done in

order to make the capacitance of the center value smaller than one half of the actual capacitance value because

a smaller capacitance value increases the frequency variation.

Figure 52 Crystal Oscillator Setting Flow

Caution 1. The oscillation frequency varies depending on the ambient temperature and power supply

voltage. Refer to " Characteristics (Typical Data)".

2. The 32.768 kHz crystal oscillator operates more slowly at an operating temperature higher or

lower than

+20°C to +25°C. Therefore, it is recommended to set the oscillator to operate

slightly faster at normal temperature.

S35390A Datasheet(PDF) 41 Page - Seiko Instruments Inc

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