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IXMS150PSI Datasheet PDF : 10 Pages
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IXMS 150
Functional Description
Introduction
The IXMS150 is designed with mono-
lithic CMOS technology. The IC is
primarily intended for use with two-
phase step motors in the microstepping
mode but may also be used for control
of two DC motors, audio amplifiers, or
any application requiring two synchro-
nized PWMs. The IXMS150 simulta-
neously controls the currents in each of
two separate H-bridges. This device
utilizes both analog and digital functions.
The IC has five fundamental sections:
(1) oscillator and feedforward circuitry,
(2) analog section for control of the
motor currents, (3) a protection network
to protect the H-bridges and the motor
from abnormal conditions, (4) the digital
PWM logic for the control signals, and
(5) the power supply section which
includes a negative bias generator.
Oscillator
The IXMS150 contains an internal
oscillator which is controlled by
adjusting the values of RO and CO.
These two components determine the
switching frequency, amount of dead
time, and the minimum pulse width at
output pins 9, 10, 15 and 16. The
minimum and maximum values of RO
and CO are given in the Electrical
Characteristics.
The oscillator also sets the frequency of
the charge pump circuit in the internal
negative bias generator (VBB). At lower
frequencies (<40 kHz) the value of
CPUMP must be increased to assure
proper operation.
Feedforward Compansation
In all fixed frequency PWM control
systems open loop gain, motor current
slew rate, and motor current ripple are
proportional to the motor supply
voltage. Gain variations due to supply
voltage changes complicate the design
of such systems and restrict their band-
width to the minimum worst case
condition. For this reason, an advanced
adaptive compensation scheme is built-
in using a feedforward technique. This
feature has been designed such that
open loop gain is inversely proportional
to the voltage applied to the FFWD pin,
normally a fraction of the motor supply.
As a result, open loop gain can be
I - 38
made independent of the high voltage
supply and system bandwidth can be
maximized.
Analog Section
The analog section of each channel of
the IXMS150 consists of a signal
processor and an error amplifier. The
signal processor is required since the
voltage developed across the sense
resistor often contains transients asso-
ciated with the switching characteristics
of the power devices. These transients
need to be properly filtered for the
system to operate with the desired
degree of precision. Because of this,
the IXMS150 uses proprietary analog
and digital signal processing techni-
ques that sense the true average phase
currents. Since this requires only one
sense resistor per H-bridge it avoids
mismatches in charge/discharge
currents associated with two sense
resistor per H-bridge topologies.
The instantaneous difference between
the motor current and the control input
is integrated via the E/A amp and fed to
the PWM comparator to generate the
appropriate signals for the H-bridges.
External compensation of the input and
sense signals is provided for via the
comp1, comp2 and comp3 pins.
Protection Circuitry
The IC has a two-level Over/Excess
Current protection circuit. Maximum
current is represented as 0.625 V at the
SENSE input. If the SENSE voltage
exceeds 0.9 volts for more than one
microsecond, the switching outputs
(VOUT) and OUTDIS will be forced low.
This represents a current that is 40 %
beyond full scale. If the SENSE voltage
exceeds 3.6 V, these outputs will be
forced low immediately. This repre-
sents a current that is 500% beyond full
scale. The time delay on the lower level
of overcurrent avoids erroneous
shutdowns as a result of noise spikes
that are coupled from the motors H-
bridges. Note that the threshold
voltages cited here assume a supply of
+12 V.
Undervoltage Lockout
A third protection mechanism is the
Under-Voltage Lockout. It assures
proper behavior on power-up and
power-down and avoids high power
dissipation in the H-bridge due to
insufficient gate voltage. It uses a zener
for reference and has a trip point set at
8 V. It will also check to make sure
there is sufficient negative bias to
insure proper operation. This is typically
-1.6 V. OUTDIS will be held low by the
UV Lockout circuit until VBB and VDD
reach these values.
Output Disable Feature
To enable external over-temperature
protection, the output disable pin
(OUTDIS) is available on the IXMS150.
When pulled low this disables the
output by forcing all output pins low.
The same output disable input pin is
also used as a status output. When it is
pulled low by the internal circuitry it
indicates an error condition such as
undervoltage (VDD), insufficient negative
bias voltage (VBB) or over/excess
current. This can be used as a status
indicator in smart systems.
PWM Section
The PWM comparator generates two
complementary signals based on the
output of the error amplifier. Dead-time
is then added which is adjusted by the
selection of the external oscillator
capacitor. There is also a minimum duty
cycle clamp circuit that allows the use
of an AC coupled H-bridge.
Supply Section
The main power supply (VDD) is applied
to pin 24. This is typically +12 V. Internal
bias circuitry presents a VDD/2 reference
voltage at pin 8, BYPASS. A 0.1 µF
capacitor should be connected from pin
8 to analog ground for noise immunity.
Negative Bias Generator
The IXMS150 samples both positive
and negative voltages at the motor
sense feedback resistor. In addition,
since errors in the input current around
zero are a major contributor to micro-
step positioning error, the input control
range is bipolar and specified as ±2 V
full scale. For these reasons it is desi-
rable to have both positive and nega-
tive power supplies. In order to enable
single 12 V supply operation, a negative
voltage generator and regulator are
built into the IC. This is a charge pump
circuit whose frequency is that of the
onboard oscillator. It utilizes an external
pair of capacitors and diodes to gene-
rate a negative bias equal to -VDD/5 or
approximately -2.4 V for VDD = 12 V.
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