Cross conduction protection (see Fig. 7) avoids simultaneously turning on both the upper and lower

DMOS of each half bridge. There is a fixed delay time of 300ns between the turn on and the turn off of

the two DMOS switches in each half bridge. The switching operating frequency is up 50kHz. High com-

mutation frequency permits the reduction of ripple of the output current but increases the device’s power

dissipation, however low commutation frequency causes high ripple of the output current. The switching

frequency should be higher than 16kHz to avoid acoustic noises.

The sink current at the INPUTS and ENABLES pins is approximately 30

µA if the voltage to these pins is

at least 1V less than the Vref voltage (see Fig. 3 and Fig. 4). To avoid overload of the logic INPUTS and

ENABLES , voltage should be applied to Vs prior to the logic signal inputs.

POWER DISSIPATION

An evaluation of the power dissipation of the IC driving a three phase motor in a chopping current con-

trol application follows.

With a simplified approach it can be distinguished three periods (see Fig. 8) :

Rise Time, Tr, period.

This is the rise time period, Tr, in which the cur-

rent switches from one winding to another. In this

time a DMOS is switched on and the current in-

creases up to the peak value Ipk with the law i(t)

= (Ipk/Tr) t. The energy lost for the rise time in

the period T is :

Erise

=

∫

0

Tr

Rdson

Tr

3

Fall Time,Tf, period.

When the current switches from one winding to

another, there is a fall time in which the current

that flows in the intrisic diode of the DMOS de-

creases from Ipk to zero. If VD is the voltage fall

of the diode, the energy lost is :

Efall

=

∫

0

tf

VD

(t) ⋅ i(t)dt

Tload

During this time the current that flows in the winding is limited by the chopping current control. The en-

ergy dissipated due to the ON resistance of the DMOS is :

Eload

In the formula, Irms is the RMS load current, given by :

Irms

=

√







− Ival

√3







2

and Iload is the average load current.

When the switch is ON, the energy dissipated due to the commutation of the chopping current control in

the DMOS can be assumed to be:

Eon

= Vs ⋅ Ival ⋅

tcom

2

where tcom is the commutation time of the DMOS switch.

Trise

Tfall

Tload

Tchop

Ipk

Ival

Iload

Figure 8.

AN1088 APPLICATION NOTE

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