Diodes ZXBM5210 - Manual (Page 7)

ZXBM5210
Document number: DS36765 Rev. 1 - 2
7 of 17
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December 2013
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ZXBM5210
Application Note
(cont.)
Motor Speed Control with a PWM Input Signal (cont.)
Depending on the motor design and its inertia the minimum start-up PWM duty required can be typically between 30% - 50%. While the motor is
rotating minimum PWM duty can generally be reduced down to 20%. How small the PWM duty can be without stalling the motor depends on the
motor mechanical and coil design parameters and not limited by the output capability of the device. If voltage at V
DD
is lower than the nominal
motor voltage, both start-up PWM duty and minimum running PWM duty required will be higher.
Motor Speed Control by DC Supply Voltage
Motor speed can be controlled by varying the V
DD
supply voltage while the FWD and REV pins are set to either a logic high or low depending on
forward or reverse direction needed. The V
REF
must be equal to V
DD
in this mode. For example, if the V
DD
for a 12V motor is changed from
12V to 3V the speed will be reduced from 100% to 25%.
Re-Circulating Capacitor
During motor operation when the low side switch is turned off the bridge or the motor voltage may overshoot to high levels if there is no current
path for the energy in the motor to flow. Such high voltages can damage the IC. A current path can be provided by adding a bypass capacitor
from the V
DD
or V
M
to the GND. The value of the bypass capacitor depends on the motor coil design, motor current, motor voltage and the
IC voltage limits. This could be in the range of 0.47µF for low current applications to 10uF or more for large current applications.
Dead-Time
During motor current reversal (for motor rotation direction reversal), switch position changes between S1-S2 and S3-S4. Such change may result
in cross conduction between high side and low side MOSFETS, e.g. S1 and S4 or S2 and S3. To prevent cross conduction the IC provides a
dead time 3µs typical during current reversal or fast turn on of the low side MOSFETs.
Back-Rush Voltage
Depending on motor characteristics, the environment and the ambient conditions back-rush voltage (at the bridge) may fluctuate during brush
commutation and PWM switching. Due to the energy in the coil this back-rush voltage can reach high levels if no adequate alternative current
path is provided when inductor current path is interrupted. The back-rush voltage overshoot should not be allowed to go beyond the operating
voltage range of the IC. This backrush voltage overshoot can be minimized by using a re-circulating bypass capacitor at the V
DD
and V
M
pins.
The value of the re-circulating bypass capacitor depends on the motor coil design, motor current, motor voltage and the IC voltage limits. This
could be 0.47µF for low current applications to 10uF or more for large current applications.
Under Voltage Lockout
To make sure the minimum voltage needed to operate the driver is supplied, the driver has an under voltage lock out. At start up the device will
only start if the supply voltage is typically 2.8 or greater. During normal operation, the device will switch off all the output switches and power
down if the supply voltage drops below 2.6V typical.
Over Voltage Protection
When the supply voltage exceeds 20.7V (typical) the driver will turn-off all the output switches. The driver will return to normal condition if the
supply voltage drops below 19V (typical) provided no other fault condition or signals are preventing it to enter normal operation.
In-Rush Current
It is recommended to use the PWM duty cycle to control the average voltage supplied to the motor during power up, standby mode, brake mode
or during motor direction reversal. If a PWM signal is not available it is recommended to use a current limiting resistor or other protection
devices if needed.
Over-Current Setting and Protection
The internal over current protection (OCP) threshold is 1.5A typical at 12V supply +25°C.
When the motor current exceeds the OCP threshold for longer than 10µs typical on any of the H-Bridge switches, the device will switch of all the
output switches and remain off for 5ms typical. The IC returns to normal operation after the 5ms if over current condition has gone away. If the
motor current is still higher than the OCP threshold, the device will enter another 5ms standby mode.
Thermal Shutdown
The device has an internal thermal shutdown to prevent a thermal run-away scenario. The thermal shutdown is triggered when the junction
temperature of the device reaches +165°C. It will remain in standby mode until the junction temperature falls by +25°C.
Reverse Voltage Protection
If reverse protection is needed this can be achieved by adding an external diode to the V
DD
and V
M
pins.