ML4428
COMPONENT SELECTION GUIDE
In order to properly select the critical components for the
ML4428 you should know the following things:
1. The motor operating voltage, V
MOTOR
(V).
2. The maximum operating current for the motor,
I
MAX
(A).
3. The number of poles the motor has, N.
4. The back-EMF constant of the motor, K
e
(V
¥
s/rad).
5. The torque constant of the motor, K
e
(N
¥
m/A). (This
is the same as the back-EMF constant, only in
different units.)
6. The maximum desired speed of operation, RPM
MAX
(rpm).
7. Line to line resistance, R
L-L
(Ohms).
8. Line to line inductance, L
L-L
(Henries).
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.
R
SENSE
The function of R
SENSE
is to provide a voltage proportional
to the motor current, for current limit/feedback purposes.
The trip voltage across R
SENSE
is 0.5V so:
R
SENSE
=
0.5
I
MAX
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 8700�½. This requires the external resistor to
be set as follows and results in the given attenuation.
RES1 = RES2 = RES3
RESI = 725 (V
MOTOR
– 10)
2900
RES1
+
8700
A larger value for RES1 may be required if the peak motor
phase voltage exceds V
MOTOR
.
Atten
=
I
SENSE
FILTER
The I
SENSE
filter consists of an RC lowpass filter in series with
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 = 1K�½ 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.
If excessive noise is present on the I
SENSE
pin, the capacitor
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.
C
IOS
The one shot capacitor determines the off time after the
current limit is activated, i.e. the voltage on the I
SENSE
pin
exceeded 0.5V. The following formula ensures that the
motor current is stable in current limit:
C
IOS(MAX)
=
1.11
×
10
−
11
×
V
MOTOR
C
IOS
is in Farads
This is the maximum value that C
IOS
should be. Higher
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)
I
MAX
is the maximum motor current.
The power dissipation in the resistor is I
MAX
squared times
R
SENSE
, so the resistor should be sized appropriately. For
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.
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
