UC1638
UC2638
UC3638
APPLICATION INFORMATION
UDG-95049-2
Figure 1. Average Motor Current Control Loop
The UC3638 is designed to provide pulse width modula- sults in a voltage null point of zero volts. For this condi-
tion control of DC brush motors in applications requiring tion, AREFIN should be tied to the negative voltage sup-
precision torque, velocity, or position control. Due to its ply rail (VEE), which disables the internal voltage buffer,
high frequency capability, other high power applications allowing AREF to be tied to ground. For a single supply
such as switch mode audio amplifiers can also be ad- system, AREFIN should be left open circuit, and AREF
dressed. Through a combination of circuit sophistication should be decoupled to VEE (system ground) with at
and integration, the designer can maintain a high level
of flexibility, while reducing cost compared to solutions
using other PWM ICs.
least 0.1µF. The resulting voltage null point for this case
Figure 1 shows a typical application circuit for the
UC3638. By taking advantage of the UC3638's many in-
tegrated functions, a low cost and compact average cur-
rent mode motor controller can be designed. Depending
on the level of complexity, as many as 15 discrete com-
ponents and an additional high bandwidth amplifier can
be saved compared to a similar circuit using the
UC3637 PWM controller.
Oscillator Section and Modulation Scheme
Figure 2 depicts the UC3638 oscillator and PWM wave-
forms for the condition where the output of the voltage
amplifier (COMP) is at the null point (same voltage as
AREF). For applications using split voltage supply rails,
AREF will normally be tied to system ground. This re-
UDG-95068-1
Figure 2. Oscillator and PWM Waveforms
6
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