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STA6940M

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18
7
SANKEN ELECTRIC CO., LTD.
28106.01
Brushed DC Motor Driver IC With PWM Control
STA6940M
Constant-Current Control (PWM)
When the motor is starting up, the current, I
M
, increases to the set
current limit, as shown in figure 2. During operation, the motor
current, I
M
, is monitored using the voltage across resistor R
S
,
which is compared to the set point V
PREF
.
When the current limit is reached, the device turns-off the high-
side MOSFET and body-diode of the low-side MOSFET allows
the back-EMF current to flow in the coil for 35 μs (Slow Decay
mode), as shown in figure 3. After this time expires, the PWM
control reverts to on. There is a blanking time of 5 μs during
turn-on to prevent malfunction due to noise surges. During the
blanking time, the current control does not operate, which means
that the minimum on-time is also the blanking time.
Mot or
VM
OA OB
ON
m
OFF OFF
OFF ON
IM_ON
Rs
IM_OFF
Slow Decay: 35 μs
Figure 3. Currrent Control. The current path as current is rising is shown by
the solid arrow (IM_ON), the off-time current is shown by the dashed arrow
(IM_OFF).
Figure 2. Currrent Control Waveforms. As can be seen at the points (A), PWM
timing and Diag pin oscillation are NOT synchronous.
0
I
M
0
V
PREF
V
RS
I
PREF
Off-Time
35 s
Blanking Time
5 s
0
(A) (A)
Diag
8
SANKEN ELECTRIC CO., LTD.
28106.01
Brushed DC Motor Driver IC With PWM Control
STA6940M
Phase PWM Control
The current-control method uses fixed off-time and blanking time,
as detailed above. Even when the PWM_REF terminal is at a low
voltage, however, the current will still flow, and increase, dur-
ing the blanking time (minimum on-time). This minimal current,
because of the fixed off-time, will only decay to a certain point.
To enable current control below this minimal current level, the
Phase PWM control method must be used. This is different from
the PWM current control as detailed above, and external PWM
signals with the correct on/off duty cycle must be used.
Phase PWM control has two modes, Fast Decay mode and Slow
Decay Mode.
Fast Decay Mode This mode uses the Free motor state. In other
words, during recirculation time, the IN1 and IN2 pins are both
set low. During motor driving time, the current control point is
determined by the duty cycle and frequency of the input signals:
• The input PWM signals must have an on-duty cycle greater than
50% for proper operation.
• The input PWM signals should have a recommended frequency
of 30 to 50 kHz.
Motor rotation direction is determined by pulsing one or the other
logic input, as shown in table 3. For the forward direction, current
flows are shown in figure 4, and the input pulse and resulting cur-
rent pulsing is shown in figure 5.
Another new method of control during recirculation, is to change
the inputs shown in table 3, applying low signals instead of high
(the pulsed signals remain as in the table). This method allows the
driver to dissipate less heat by turning on the MOSFET for the
back-EMF current decay instead of using the body diodes. This
method is also known as Synchronous Rectifier control.
Table 3. Input Truth Table, Phase PWM Control
Input
Motor Function
IN1 IN2
Fast Decay Mode
PWM pulse Low Forward
Low PWM pulse Reverse
Slow Decay Mode
PWM pulse High Forward
High PWM pulse Reverse
ON
m
OFF
ON
m
OFF
Mot or
VM
OA OB
OFF
OFF
IM_ON
Rs
IM_OFF
Figure 4. Phase PWM control (Fast Decay mode), current flows shown are
for the forward direction
Figure 5. Phase PWM control (Fast Decay mode), logic input pulsing
shown for the forward direction
IN1
IN2
IM_ON
IM_OFF
IM_ON
+
IM_OFF
9
SANKEN ELECTRIC CO., LTD.
28106.01
Brushed DC Motor Driver IC With PWM Control
STA6940M
Slow Decay Mode This mode uses the Brake motor state. In
other words, during recirculation time, the IN1 and IN2 pins are
both set high. During motor driving time, the current control
point is determined by the duty cycle and frequency of the input
signals:
• The input PWM signals must have an on-duty cycle less than
50% for proper operation.
• The input PWM signals should have a recommended frequency
of 30 to 50 kHz.
Motor rotation direction is determined by pulsing one or the other
logic input, as shown in table 3. For the forward direction, current
flows are shown in figure 6, and the input pulse and resulting cur-
rent pulsing is shown in figure 7.
Overcurrent Protection (OCP)
In the STA6940M, the overcurrent protection feature is designed
to protect against rotor lock or coil short conditions. This protec-
tion is triggered when the motor current, I
M
, as detected by the
resistor R
S
, reaches the set level of OCP_REF.
When the OCP threshold is reached, the driver turns-off all
MOSFETs for 135 μs (figure 8). The decaying current must flow
through the body diodes to the main supply (Fast Decay mode),
as shown in figure 9.
Note: OCP operation does not disable the driver. OCP is flagged
on the Diag pin, and the system logic of the application should
control the response.
OFF
m
ON
ON
m
OFF
Mot or
VM
OA OB
OFF
ON
IM_ON
Rs
IM_OFF
ON
m
OFF
ON
m
OFF
Mot or
VM
OA OB
OFF
OFF
IM_ON
Rs
IM_OFF
Fast Decay: 135 μ s
Figure 6. Phase PWM control (Slow Decay mode), current flows shown are
for the forward direction
Figure 8. OCP operation (Fast Decay mode)
Figure 7. Phase PWM control (Slow Decay mode), logic input pulsing
shown for the forward direction
Figure 9. OCP operation waveforms (Fast Decay mode)
IN1
IN2
IM_ON
IM_OFF
IM_ON
+
IM_OFF
0
I
M
V
RS
0
V
OREF
I
OREF
Off-time
135 s
0
Diag
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18

STA6940M

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Brushed DC Motor Driver 18-Pin ZIP Tube
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