ML4425CH Micro Linear, ML4425CH Datasheet - Page 7

• Manufacturer Part Number: ML4425CH
• Description: Sensorless BLDC Motor Controller
• Manufacturer: Micro Linear

FUNCTIONAL DESCRIPTION
CURRENT LIMITING IN THE POWER STAGE
The current sense resistor (R
regulates the maximum current in the power stage and
the BLDC motor. Current regulation is accomplished by
shutting off the output drivers LA, LB, and LC for a fixed
amount of time if the voltage across R
current limit threshold.
I
The voltage on the I
threshold. The ML4425 has an internal voltage divider
from V
2.3V (see Figure 2). An external voltage divider
referenced to V
I
times the current flow of the internal divider.
R
The function of R
proportional to the motor current to set the current limit
trip point. The default trip voltage across R
460mV, set by the internal I
sense resistor should be a low inductance resistor such as
a carbon composition. For resistors in the milliohms range,
wire-wound resistors tend to have low values of
inductance. R
dissipation (I
I
The I
current sense signal as shown in Figure 2. The purpose of
this filter is to remove the diode reverse recovery
shootthrough current. This current causes a voltage spike
on the leading edge of the current sense signal which
may falsely trigger the current limit. The current sense
voltage waveform is shown before and after filtering in
Figure 3. The recommended starting values for this circuit
are R = 1k
330ns, and will filter out spikes of shorter duration. C can
be increased to as much as 2.2nF, but should not exceed
a time constant of more than a few microseconds.
C
When I
shot is activated, turning off LA, LB, and LC for a fixed
amount of time (t
capacitance connected to C
fixed off time equal to or less than the PWM period. For a
25kHz PWM frequency, the PWM period is 40µs; t
should be between 20µs and 40µs. The lower limit of t
is dictated by the minimum on time of the power stage; a
safe approximation is 5µs or less. The equation for finding
the C
LIMIT
SENSE
LIMIT
SENSE
IOS
C
SENSE
IOS
setting. The external divider should have at least 10
OS
REF
Filter
SENSE
=
capacitance value is as follows:
that sets a default current limit threshold of
RC lowpass filter is placed in series with the
t
OFF
and C = 330pF. This gives a time constant of
MAX 2
exceeds 0.2
SENSE
2 4
REF
™ 50
.
SENSE
OFF
V
can be used to override the default
should be sized to handle the power
m
LIMIT
). t
R
A
SENSE
is to provide a voltage
OFF
pin sets the current limit
).
is set by the amount of
SENSE
I
IOS
LIMIT
LIMIT
. C
) shown in Figure 1
, the current limit one-
divider ratio. The current
IOS
is usually set for a
SENSE
SENSE
exceeds the
(Continued)
is
OFF
OFF
(1)
COMMUTATION CONTROL
A 3-phase BLDC motor requires electronic commutation
to achieve rotational motion. Electronic commutation
requires the switching on and off of the power switches of
a 3-phase half bridge. For torque production to be
achieved in one direction, the commutation is dictated by
the rotor position. Electronic commutation in the ML4425
is achieved by turning on and off, in the proper sequence,
one N output from one phase and one P output from
another phase. There are six combinations of N and P
outputs (six switching states) that constitute a full
commutation cycle. These combinations are illustrated in
Table 1 and Figure 4, and are labeled states A through F.
This sequence is programmed into the commutation state
machine. Clocking of the commutation state machine is
provided by a voltage controlled oscillator (VCO).
R SENSE
FROM
460mV
0V
Figure 3. Current Sense Resistor Waveforms
(a) Without Filtering, and (b) With Filtering
Figure 2. Current Sense Circuitry
V REF
(a)
I SENSE
I LIMIT
16k
V REF
5
8k
2.9V
0V
+
–
C IOS
ML4425
STOP
START
(b)
S
R
Q
Q
ON/OFF
30µA
PWM
7