Datasheet

TMC246 DATA SHEET (V2.02 / Jul. 31st, 2006) 18

Oscillator Capacitor

The PWM oscillator frequency can be set by an external capacitor. The internal oscillator uses a 28kΩ

resistor to charge / discharge the external capacitor to a trip voltage of 2/3 Vcc respectively 1/3 Vcc. It

can be overdriven using an external CMOS level square wave signal. Do not set the frequency higher

than 100kHz and do not leave the OSC terminal open! The two bridges are chopped with a phase shift

of 180 degrees at the positive and at the negative edge of the clock signal.

[nF] C s40

OSC

≈

OSC: PWM oscillator frequency

COSC: Oscillator capacitor in nF

Table of oscillator frequencies

OSC

typ. C

OSC

16.7kHz 1.5nF

20.8kHz 1.2nF

25.0kHz 1.0nF

30.5kHz 820pF

36.8kHz 680pF

44.6kHz 560pF

Please remark, that an unnecessary high frequency leads to high switching losses in the power

transistors and in the motor. For most applications a chopper frequency slightly above audible range is

sufficient. When audible noise occurs in an application, especially with mixed decay continuously

enabled, the chopper frequency should be two times the audible range. For most applications we

recommend a frequency of 36.8kHz.

Pullup resistors on unused inputs

The digital inputs all have integrated pull-up resistors, except for the ENN input, which is in fact an

analog input. Thus, there are no external pull-up resistors required for unused digital inputs which are

meant to be positive.