Digilent 410-185P-KIT - Manual (Page 5)

Cerebot MC7 Reference Manual
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provide the transistors and a Microchip
MCP14700MF FET driver. The FET driver
provides a high side transistor drive input and
a low side transistor drive input.
The NTMD4820N FETs are rated for a
maximum V
DS
of 30V, maximum V
GS
or 20V,
and maximum drain current (I
D
) of 8A at 25°C,
6.4A at 70°C (t<10s) or 6.4A continuous drain
current at 25°C and 5.1A at 70°C. For more
detailed information on the NTMD4820N, refer
to the device data sheet available at the On
Semiconductor web site.
The steady state temperature of the FETs with
all four half bridges switching at 50% duty
cycle and a load current of 5A each has been
measured to be ~55°C. Operation in some
applications may require heat sinking or fan
cooling of the board to maintain continuous
operation at the full 5A output current.
The high side and low side drive inputs for
each half bridge are connected to PWMH and
PWML outputs from the Motor Control PWM
module in the dsPIC microcontroller. The
Motor Control PWM module is an advanced
pulse width modulation generator optimized for
motor control applications. This PWM unit
provides eight outputs arranged as four high
and low side drive pairs for controlling four half
bridge circuits.
The half bridges can be operated
independently, in pairs to form two h-bridges,
or in other combinations using the Motor
Control PWM module.
Connector J20 is used to provide the power
supply voltage for the high side of the half
bridges, and therefore powers whatever
external load is being controlled by the board.
This is a screw terminal connector on the left
side of the board. A power supply or battery of
5V-24V can be used, with a maximum
continuous current of 10A.
The half bridge connections between the
bridge output signals and the Motor Control
PWM module are configured in the following
way:
VMOT1 PWM1H, PWM1L
VMOT2 PWM2H, PWM2L
VMOT3 PWM3H, PWM3L
VMOT4 PWM4H, PWM4L
These outputs from the bridges are available
on connectors J14 & J15 or J16 & J18.
Connectors J14 & J15 are screw terminal
connectors for wire attachment. Connectors
J16 & J18 are 6-pin JST connectors used by
the gear head motors available from Digilent.
Connectors J14 & J16 provide the outputs
VMOT1 and VMOT2, while J15 & J18 provide
the outputs VMOT3 and VMOT4.
Current and Voltage Feedback
Each of the four half bridges is configured for
monitoring the current flow through the low
side transistor of the bridge. A voltage
proportional to the bridge current is applied to
one of the A/D inputs on the dsPIC
microcontroller for each half bridge.
A 10 milli-ohm (0.01 ohm), 1% current sense
resistor is connected between the source of
each low side FET and ground. A differential
amplifier with a fixed gain of 21 and an offset
voltage of 1.0V is used to buffer and amplify
the voltage across the current sense resistors
before being applied to the inputs to the A/D
converter. The following formula relates the
voltage applied to the A/D converter input to
the sensed bridge current:
V = (0.21 * I) + 1.0
Refer to Appendix C for tables showing data
measured from an example board.
In addition to the bridge current, the output
voltage of each half bridge can be monitored
as well. The output voltage of each half bridge
can be connected via a voltage divider to one
of the inputs to the A/D converter. The voltage
divider ratio is as follows:
Vo = 2.2/17.2 * Vi = 0.128 * Vi