ML4425 查看數據表（PDF） - Micro Linear Corporation
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ML4425 Datasheet PDF : 16 Pages
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FUNCTIONAL DESCRIPTION (Continued)
CURRENT LIMITING IN THE POWER STAGE
The current sense resistor (RSENSE) shown in Figure 1
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 RSENSE exceeds the
current limit threshold.
ILIMIT
The voltage on the ILIMIT pin sets the current limit
threshold. The ML4425 has an internal voltage divider
from VREF that sets a default current limit threshold of
2.3V (see Figure 2). An external voltage divider
referenced to VREF can be used to override the default
ILIMIT setting. The external divider should have at least 10
times the current flow of the internal divider.
RSENSE
The function of RSENSE is to provide a voltage
proportional to the motor current to set the current limit
trip point. The default trip voltage across RSENSE is
460mV, set by the internal ILIMIT divider ratio. The current
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. RSENSE should be sized to handle the power
dissipation (IMAX2 ´ RSENSE).
ISENSE Filter
The ISENSE RC lowpass filter is placed in series with the
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.
CIOS
When ISENSE exceeds 0.2 ´ ILIMIT, the current limit one-
shot is activated, turning off LA, LB, and LC for a fixed
amount of time (tOFF). tOFF is set by the amount of
capacitance connected to CIOS. CIOS 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; tOFF
should be between 20µs and 40µs. The lower limit of tOFF
is dictated by the minimum on time of the power stage; a
safe approximation is 5µs or less. The equation for finding
the CIOS capacitance value is as follows:
COS
=
t OFF 50mA
2.4V
(1)
ML4425
FROM
RSENSE
VREF
ISENSE
×5
–
+
VREF
16kΩ
ILIMIT
2.9V
0V
8kΩ
PWM
ON/OFF
SQ
RQ
STOP
START
CIOS
30µA
Figure 2. Current Sense Circuitry
460mV
0V
(a)
(b)
Figure 3. Current Sense Resistor Waveforms
(a) Without Filtering, and (b) With Filtering
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).
7
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