7

ML4428

RES1, RES2 and RES3

Operating motors at greater than 12V requires attenuation

resistors in series with the sense inputs (PHI1, PHI2, PHI3)

to keep the voltage less than 12V. The phase sense input

impedance is 8700ý. This requires the external resistor to

be set as follows and results in the given attenuation.

RES1 = RES2 = RES3

Atten

RES

=

+

2900

1 8700

A larger value for RES1 may be required if the peak motor

the current sense signal. The purpose of this filter is to filter

out noise spikes on the current, which may cause false

triggering of the one shot circuit. It is important that this filter

not slow down the current feedback loop, or destruction of

the output stage may result. The recommended values for

this circuit are R = 1Ký and C= 300pF. This gives a time

constant of 300ns, and will filter out spikes of shorter

duration. These values should suffice for most applications.

may be increased at the expense of speed of current loop

response. The filter time constant should not exceed 500ns

or it will have a significant impact on the response speed of

the one shot current limit.

The one shot capacitor determines the off time after the

exceeded 0.5V. The following formula ensures that the

motor current is stable in current limit:

CV

IOS MAX

MOTOR

()

.

=×

×

−

111 10

11

average torque during the current limit cycle can be

achieved by reducing this value experimentally, while

monitoring the motor current carefully, to be sure that a

runaway condition does not occur. This runaway

condition occurs when the current gained during the on

time exceeds the current lost during the off time, causing

the motor current to increase until damage occurs. For

most motors this will not occur, as it is usually a self

limiting phenomenon. (See Figure 7)

COMPONENT SELECTION GUIDE

In order to properly select the critical components for the

ML4428 you should know the following things:

2. The maximum operating current for the motor,

3. The number of poles the motor has, N.

is the same as the back-EMF constant, only in

different units.)

(rpm).

9. The motor should have at least 15% line-to-line

inductance variation during rotation for proper start-

up sensing. (Air core motors will not run using the

ML4428.) Examine the motor to determine if there is

any iron in the core. If the stator coils are not wound

around an iron form, the ML4425 or ML4426 may be

a better choice.

If you do not know one or more of the above values, it is

still possible to pick components for the ML4428, but some

experimentation may be necessary to determine the

optimal value. All quantities are in SI units unless other

wise specified. The formulas in the following section are

based on linear system models. The following formulas

should be considered a starting point from which you can

optimize your application.

Note: Refer to Application Note 43 for details on loop

compensation.

to the motor current, for current limit/feedback purposes.

R

I

SENSE

MAX

= 05

.

very high current motors, a smaller resistor can be used,

with an op-amp to increase the gain, so that power

dissipation in the sense resistor is minimized.