IXMS150PSI datasheet(7/10 Pages) IXYS

7 / 10 page

I - 41

Use the formula

C2 = 100 µF 100 kHz/f

OSC

for other frequencies.

With V

DD = 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

O and CO:

OSC = 1/RO (CO + CP))

(10)

Note: C

P is a 38 pF (typ.) internal

parasitic capacitor.

IXMS 150

1/F

V(t)

2 V (PIN 7)

7 V (PIN 7 Open)

[

]

A =

1/f

OSC = Ro (Co + Cp)

Fig. 6a: Oscillator Block Diagram

Fig. 6b: Oscillator Waveform Diagram

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 R

ff1 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 C

O. The relationship is:

pw(min) = Rmp (CO + Cp)(11)

Note: R

mp is a 3.6 kΩ (typ.) internal

resistor, and C

p is a 38 pF (typ.) internal

parasitic capacitor.

Dead Time

Dead time is adjusted via the external

oscillator capacitor C

O. There is an

internal resistor in the dead time circuit

as well. The relationship is:

DT = RDT (CO + Cp)

(12)

Note: R

DT is a 1.4 kΩ (typ.) internal

resistor and C

p 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) = V

HV/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

backEMF willtend 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

1/f

OSC

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