ML4425
FUNCTIONAL DESCRIPTION (Continued)
CURRENT LIMITING INTHE POWER STAGE
FROM
PWM
ON/OFF
R
SENSE
I
I
The current sense resistor (R
) shown in Figure 1
SENSE
SENSE
× 5
–
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
S
Q
+
V
REF
V
REF
R
Q
amount of time if the voltage across R
current limit threshold.
exceeds the
SENSE
16kΩ
LIMIT
2.9V
0V
STOP
START
30µA
I
8kΩ
LIMIT
C
IOS
The voltage on the I
pin sets the current limit
LIMIT
threshold. The ML4425 has an internal voltage divider
from V that sets a default current limit threshold of
REF
2.3V (see Figure 2). An external voltage divider
referenced to V can be used to override the default
REF
I
setting. The external divider should have at least 10
LIMIT
Figure 2. Current Sense Circuitry
times the current flow of the internal divider.
R
SENSE
The function of R
is to provide a voltage
SENSE
proportional to the motor current to set the current limit
trip point. The default trip voltage across R is
460mV
SENSE
460mV, set by the internal I
divider ratio. The current
LIMIT
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
0V
inductance. R
should be sized to handle the power
SENSE
2
dissipation (I
´ R
).
MAX
SENSE
(a)
(b)
I
Filter
SENSE
Figure 3. Current Sense Resistor Waveforms
(a) Without Filtering, and (b) With Filtering
The I
RC lowpass filter is placed in series with the
SENSE
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 = 1kW 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.
COMMUTATIONCONTROL
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).
C
IOS
When I
exceeds 0.2 ´ I
, the current limit one-
LIMIT
SENSE
shot is activated, turning off LA, LB, and LC for a fixed
amount of time (t ). t is set by the amount of
OFF OFF
capacitance connected to C . C
is usually set for a
IOS
IOS
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 approximation is 5µs or less. The equation for finding
the C
capacitance value is as follows:
IOS
tOFF 50 A
COS
=
(1)
2.4V
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
