TMC246 / TMC246A DATA SHEET (V2.05 / 2009-Jul-30)
19
Oscillator Capacitor
The PWM oscillator frequency can be set by an external capacitor. The internal oscillator uses a 28k
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.
1
fOSC
40 s COSC [nF]
fOSC:
COSC:
PWM oscillator frequency
Oscillator capacitor in nF
Table of oscillator frequencies
fOSC typ. COSC
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.
Power supply sequencing considerations
Upon power up, the driver initializes and switches off the bridge power transistors. However, in order
for the internal startup logic to work properly, the Vcc supply voltage has to be at least 1.0V,
respectively, the Vs supply voltage has to be at least 5.0V. When Vs goes up with Vcc at 0V, a medium
current temporary cross conduction of the power stage can result at supply voltages between 2.4V and
4.8V. While this does no harm to the driver, it may hinder the power supply from coming up properly,
depending on the power supply start up behavior.
In order to prevent this from occurring, either use a dual voltage power supply, or use a local regulator,
generating the 5V or 3.3V Vcc voltage.
Please pay attention to the local regulator start up voltage: Some newer switching regulators do not
start before the input voltage has reached 5V. Therefore it is recommended to use a standard linear
regulator like 7805 series or LM317 or a low drop regulator or a switching regulator like the LM2595,
starting at relatively low input voltages.
Copyright © 2005, TRINAMIC Motion Control GmbH & Co KG
TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
