PRODUCT SPECIFICATION ML4425
REV. 1.0.2 7/2/01 7
Current Limiting in the Power Stage
The current sense resistor (R
SENSE
) shown in Figure 1 regu-
lates 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
SENSE
exceeds the current limit thresh-
old.
I
LIMIT
The voltage on the I
LIMIT
pin sets the current limit threshold.
The ML4425 has an internal voltage divider from V
REF
that
sets a default current limit threshold of 2.3V (see Figure 2).
An external voltage divider referenced to V
REF
can be used
to override the default I
LIMIT
setting. The external divider
should have at least 10 times the current flow of the internal
divider.
R
SENSE
The function of R
SENSE
is to provide a voltage proportional
to the motor current to set the current limit trip point. The
default trip voltage across R
SENSE
is 460mV, set by the
internal I
LIMIT
divider ratio. The current sense resistor
should be a low inductance resistor such as a carbon compo-
sition. For resistors in the milliohms range, wire-wound
resistors tend to have low values of inductance. R
SENSE
should be sized to handle the power dissipation (I
MAX
2
↔
R
SENSE
).
I
SENSE
Filter
The I
SENSE
RC lowpass filter is placed in series with the cur-
rent 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 = 1kΩ and C = 330pF.
This gives a time constant of 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
IOS
When I
SENSE
exceeds 0.2 ↔ I
LIMIT
, the current limit one-
shot is activated, turning off LA, LB, and LC for a fixed
amount of time (t
OFF
). t
OFF
is set by the amount of capaci-
tance connected to C
IOS
. C
IOS
is usually set for a fixed off
time equal to or less than the PWM period. For a 25kHz
PWM frequency, the PWM period is 40µs; t
OFF
should be
between 20µs and 40µs. The lower limit of t
OFF
is dictated
by the minimum on time of the power stage; a safe approxi-
mation is 5µs or less. The equation for finding the C
IOS
capacitance value is as follows:
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 turn-
ing 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 commu-
tation state machine. Clocking of the commutation state
machine is provided by a voltage controlled oscillator
(VCO).
C
OS
t
OFF
50µA×
2.4V
---------------------------------=
(1)
× 5
I
LIMIT
I
SENSE
FROM
R
SENSE
PWM
ON/OFF
–
+
V
REF
C
IOS
16kΩ
2.9V
0V
STOP
30µA
START
8kΩ
V
REF
S
R
Q
Q
(a)
0V
460mV
(b)
Figure 2. Current Sense Circuitry
Figure 3. Current Sense Resistor Waveforms
(a) Without Filtering, and (b) With Filtering
