IXMS150PSI Ver la hoja de datos (PDF) - IXYS CORPORATION
Número de pieza
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IXMS150PSI Datasheet PDF : 10 Pages
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IXMS 150
Use the formula
C2 = 100 µF 100 kHz/fOSC
for other frequencies.
With VDD = 12 V and an oscillator fre-
quency of 100 kHz, the bias generator
should be able to source 3 mA at -2.4 V
using these component values. This
capability may be used to power other
external circuitry as long as there is
sufficient remaining negative bias to
allow the IXMS150 to operate properly.
Impact of PWM Frequency on
System Operation
PWM switching frequency has a
pronounced effect on ripple current
through the motor windings, the resul-
ting eddy current losses in the motor,
and system efficiency. As expected,
motor current ripple goes down as
frequency increases and therefore
losses resulting from ripple currents are
also reduced. Switching frequency also
impacts losses in the power stage.
These losses are associated with the
energy necessary to turn on and off the
power MOSFEts and are proportional
to the switching frequency. In addition,
the switching frequency has a limiting
effect on maximum current loop band-
width and therefore system bandwidth
and therefore system bandwidth and
maximum motor velocity.
Oscillator
The oscillator block diagram is shown
in Fig. 6.
The frequency is set by the values of
RO and CO:
fOSC = 1/RO (CO + CP))
(10)
Note: CP is a 38 pF (typ.) internal
parasitic capacitor.
Feedforward
The amplitude of the oscillator wave-
form and overall system gain are modu-
lated by the voltage applied to the
feedforward pin (FFWD). This is nomi-
nally 3.5 V which should be divided
down from the motor high voltage
supply. This will allow system band-
width to be maximized by making
overall system gain inversely propor-
tional to the motor supply voltage.
Refer to Fig. 7 for an example of how
feedforward is connected to the motor
supply. It is recommended that a filter
capacitor be connected from FFWD to
AGND to filter noise spikes from the
motor supply. Its value should be
chosen so that the time constant of the
capacitor and the parallel combination
of Rff1 and Rff2 is such that switching
noise will be filtered but not variations
in the motor supply such as 120 Hz
ripple, etc.
Minimum Pulse Width
The minimum output pulse width can
also be modified by adjusting the oscil-
lator capacitor CO. The relationship is:
tpw(min) = Rmp (CO + Cp)
(11)
Note: Rmp is a 3.6 kΩ (typ.) internal
resistor, and Cp is a 38 pF (typ.) internal
parasitic capacitor.
Dead Time
Dead time is adjusted via the external
oscillator capacitor CO. There is an
internal resistor in the dead time circuit
as well. The relationship is:
tDT = RDT (CO + Cp)
(12)
Note: RDT is a 1.4 kΩ (typ.) internal
resistor and Cp is a 38 pF (typ.) internal
parasitic capacitor.
Fig. 7 Feedforward Connection
Diagram
Motor Slew Rate Limitations
The maximum motor velocity in a
microstepping application is determined
by the maximum rate of change of the
phase currents. Once this limit is
reached the system is slew rate
limited, at which point the peak
undistorted phase current times the
frequency of the input command is a
fixed value. The theoretical limit for the
maximum di/dt of the phase currents is
determined by the motor supply voltage
and the inductance of the motor:
di/dt (max) = VHV/Lm
(13)
The limit does not take into account the
back EMF of the motor, the bandwidth
of the current loop driving the motor, or
the minimum pulse width. The motors
back EMF will tend to reduce the voltage
applied across the motor windings,
effectively reducing the maximum slew
rate. The bandwidth of the current loop
must also be high enough so as not to
degrade system performance.
Non-Circulating Operating Mode
The IXMS150 is designed to control an
H-bridge in the non-circulating mode.
The equivalent circuit for an H-bridge is
shown in Fig. 8. In the non-circulating
Fig. 6a: Oscillator Block Diagram
© 1998 IXYS All rights reserved
V(t)
VA
11//fFOOSC
>
[ VA =
2 V (PIN 7)
7 V (PIN 7 Open)
]
1/fOSC = Ro (Co + Cp)
Fig. 6b: Oscillator Waveform Diagram
>
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