TWR-MC-LV3PH Freescale Semiconductor, TWR-MC-LV3PH Datasheet - Page 6

• Manufacturer Part Number: TWR-MC-LV3PH
• Description: Power Management IC Development Tools Low-volt 3-phase MC
• Manufacturer: Freescale Semiconductor
• Type: Motor / Motion Controllers & Driversr

Specifications of TWR-MC-LV3PH

• Rohs: yes
• Product: Tower Systems
• Tool Is For Evaluation Of: MC33937A
• Input Voltage: 12 V to 24 V
• Output Voltage: 3.3 V, 5 V
• Description/function: Part of the Tower Systems
• Dimensions: 3.54 in x 3.18 in
• Interface Type: SPI
• Output Current: 8 A

Hardware Features
At the input, pull-down resistor R26 sets logic low in the absence of a signal for the low side transistor. Open input pull-down
is important because the power transistors must stay off in the case of a broken connection or an absence of power on the
expansion board. Gate driver inputs are 3.3 V compatible. The MC33937A supplies the gate drive and also provides under
voltage hold-off and over-current. The under voltage hold-off threshold value is 8 V. MC33937A has an implemented dead-
time insertion which can be configured using SPI. The default dead-time value is typically 15 µs.
One important design decision in a motor drive is the selection of gate-drive impedance for the output transistors. Resistor
R14, R15, diode D8, and the MC33937A nominal 100 mA current-sinking capability determine gate-drive impedance for the
lower half-bridge transistor. A similar network is used on the upper half-bridge. These networks set the turn-on gate drive
impedance at approximately 100 Ω and the turn-off gate drive to approximately 100 mA. These values produce transition
times of approximately 285 ns.
Transition times of this length represent a carefully-weighed compromise between power dissipation and noise generation.
Transition times longer than 250 ns tend to use too much power at non-audible PWM rates, and transition times under 50 ns
create di/dts so large that optimal operation is difficult to achieve. The 3-phase Low Voltage Motor Control Tower Module is
designed with switching times at the higher end of this range to minimize noise.
Anti-parallel diode softness is also a primary design consideration. If the anti-parallel diodes in an off-line motor drive are
allowed to snap, the resulting di/dts can cause difficult noise management problems. In general, the peak to zero di/dt must be
approximately equal to the di/dt applied to turning off the anti-parallel diodes. The IRFR540Z MOSFETs used in this design
are targeted at this kind of reverse recovery.
3.6 Bus Voltage and Current Feedback
Bus voltage is scaled down by a voltage divider consisting of R74, R77, and R79. The values are chosen so that a 36.3 V bus
voltage corresponds to 3.3 V at output V_SENSE_DCB. V_SENSE_DCB is scaled at 91mV per volt of the DC bus voltage
and is terminated on the main elevator port. An additional output, V_SENSE_DCB_HALF, provides a reference used in
zero-crossing detection. V_SENSE_DCB_HALF is scaled at 45.5 mV per volt of the DC bus voltage and is also terminated
on the main port.
Bus current is sampled by resistor R10 and amplified in either the MC33937A’s operational amplifier or external operational
amplifier U6B. This circuit provides a voltage output suitable for sampling on AD (analog-to-digital) inputs. Both operational
amplifiers are connected as differential amplifiers for bus-current sensing with the equal gain given by:
The output voltage is shifted up by +1.65V_REF to accommodate positive and negative current swings. A ±400 mV voltage
drop across the sense resistor corresponds to a measured current range of ±8 A. AMP_OUT signal is internally connected to
the over-current comparator of the MC33937A and provides an over-current triggering function.
The shunt resistor is represented by a 0.05-ohm resistance WELWYN SMD precision resistor, the same as the phase current
measurement resistors.
3.7 Safety Functions
The MC33937A provides over-current and under-voltage functions. Bus current feedback is filtered to remove spikes, and
this signal is fed into the MC33937A current comparator input ITRIP. Therefore, when the bus current exceeds the reference
value (as set by trimmer R37), all six output transistors are switched off. After a fault state has been detected, all six gate
drivers are off until the fault state is cleared by the CLINT0 command or by switching the board off. You can then switch the
power stage on.
TWR-MC-LV3PH User’s Guide, Rev. 1, 07/2012
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Freescale Semiconductor, Inc.