DRM33395 Datasheet, PDF (40/44 Pages) Motorola, Inc – ANALOG ICs 33395 EVALUATION MOTOR BOARD DESIGNER REFERENCE MANUALv

Freescale Semiconductor, Inc.
Design Considerations
reverse battery protection, a high-side, MOSFET switch, PWM
frequencies up to 28 kHz and built-in protective circuitry to prevent
damage to the MOSFET bridge and the drive’s IC, and includes: thermal
(not used), over-current, over-voltage (>27V) and under-voltage (<4V)
shutdown. A schematic is shown in Figure 4-2.
At the input, pulldown resistors R110 thru R118 set logic low in the
absence of a signal. Open input pulldown is important, since it is
desirable to keep the power transistors off in case of either a broken
connection or absence of power on the control board. In order to accept
5-volt MCUs and 3.3-volt DSPs, a Fairchild non-inverting driver
DM74ALS1034M unifies the level of the LSEx and HSEx input signals.
This component has a minimum high-state logic 1 input voltage of
2.0 volts, and a maximum low-state logic 0 input voltage of 0.8 volts. The
SMOS EVM motor board will thus enable the connection of large number
of controller boards with various MCUs or DSPs, for example
MC68HC908, HCS12, MPC555, DSP56F8XX(E), etc. Under-voltage
lockout is not included in the gate drive. If this feature is desired, the
control board’s under-voltage detection comparator can be set for 0.85
volts.
5.4 Bus Voltage and Current Feedback Signals
Feedback signals proportional to bus voltage, phase currents and
reconstructed bus current are provided in this section.
Bus voltage is scaled down by a voltage divider consisting of R104,
R105, R106, R107, R108, and R19. The values are chosen such that a
16-volt maximum bus voltage corresponds to a 3.3-volt maximum
analog-to-digital (A/D) input. Figure 5-1 depicts the dc bus voltage
circuitry. Phase currents are sampled by resistors R101, R102 and R103
in Figure 4-2 and amplified by the circuit in Figure 5-1, which shows the
circuitry for phase A. This circuit provides a voltage output suitable for
sampling with A/D inputs. An MC33502 is used as a differential amplifier.
The gain is given by:
A = R301/R302
DRM33395 Designer Reference Manual
33395 Evaluation Motor Board
38
Design Considerations
MOTOROLA
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