AA51890 Datasheet, PDF (4/7 Pages) Agamem Microelectronic Inc. – SERVO MOTOR CONTROLLER

PRELIMINARY
AA51890 _{Agamem Microelectronics Inc.}
SERVO MOTOR CONTROLLER
■ FUNCTIONAL DESCRIPTION
(1) Voltage Regulator
This circuit is composed of a band gap circuit, and outputs a 2.5-volt temperature
compensated reference voltage. This reference voltage is stabilized when the supply voltage
is variable.
(2) Sawtooth wave & One-Shot Generator
Connect a capacitor to CT pin will generate a triangular wave by constant current charging. A
typical value is 0.1uF. The constant current is determined by a resistor connected to RT pin. A
typical capacitor of approximately 0.1uF should be connected in parallel with the resistor to
increase stability.
(3) Servo Position & Over-Time logic
This circuitry consists of a pulse width comparator, which compares the servo position
detection signal (POT pin) from the one-shot timer (CT pin) whose period depends on the
resistance of a potentiometer connected to the servo's drive shaft. This feedback is what
provides the stability for the control circuitry.
(4) Pulse Stretcher
The difference between the servo control signal and the feedback signal is the error signal.
This error signal is used to toggle the direction the current flows through the servo. The
function of this pulse stretcher is to “stretch” the small error signal long enough and increases
the duty cycle to the motor for it can maintain sufficient holding force. The circuit also
implements a “dead band” function that prevents servo jitter and hunting. This is a range over
which differences between the input and reference signals will not cause servo operation.
When the signal differences exceed this “dead band” range, drive to the motor occurs. Servo’s
drive shaft. The dead band will change according to the value of resistor connected to the RDB
pin.
(5) Directional Logic Control
The comparator circuit compares pulses from the servo control input (VIN pin) and the voltage
controlled pulse generator and provides either a positive or a negative output depending on
whether the signal pulse width is larger or smaller than the position generator’s output pulse
(POT pin). Application of this error voltage to the motor driver circuit causes the motor to turn in
a direction that will minimize the error until both pulse widths are the same.
(6) PWM Control
This signal generated by directional logic control block is used to control a flip-flop that toggles
the direction the current flows through the motor. The outputs of the flip-flop drive an external
H-Bridge output driver that handles the high current going through the motor. This output of
this block will be turned on or off with each input pulse based on the status of the directional
©Copyright Agamem Microelectronics Inc.
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