SCREENSHOT 3. A fixed voltage regulator would work well here. Again, I have provided
a tinker knob for those of you that live to tinker.
any power resistor type that is composed
of a piece of resistive wire wound in a
spiral inside of its package. I chose the
Vishay/Dale WSL2512 series. These
resistors are rated for one watt and have
a tolerance of 1%. I did not experience
any noticeable heating of the WSL2512
sense resistors and the L6208PD
datasheet calls out one watt resistors.
However, if you choose to, you can put a
two watt resistor in the R9 and R10 seats.
If you go with the two watt
resistor (WSL2816), you’ll need to
rework the PCB’s H-bridge sense
resistor pads to fit the slightly larger
SMT package. (The Digi-Key part
number of the one watt WSL2512
resistors I used is WSLG-.20CT-ND.)
A pair of 1N4148 switching diodes
can be used in place of the integrated
diode pair labeled as D1. In addition to
being more compact, the single SOT-323
diode package used here is easier to handle. The BAT46SW device is a pair of
back-to-back diodes that are rated
individually with a breakdown voltage of
100V and a maximum forward current of
150 mA. Recall from our discussion last
month that D1, C1, R1, and C9 make up
a charge pump that drives the high side
MOSFETs of the L6208PD’s H-bridges.
The L6208PD charge pump will
provide a voltage to the H-bridge
MOSFET gates that is about 10 volts
higher than the incoming bulk voltage.
Even at the maximum bulk input
52 SERVO 03.2008
voltage of 52 volts, the 100V rating on
the charge pump capacitors is more
than adequate for any application.
Resistors R3, R4, and capacitor C5
form a combination voltage divider/low-pass filter for input VREFA. Capacitor C6,
along with resistors R5 and R6, perform
the same function for the L6208PD’s
VREFB input. The output current of the
H-bridge pair is directly affected by the
voltages at the VREFA and VREFB inputs.
So, with our setup we can provide a
steady state DC voltage to the VREF
inputs or use a PWM or pair of PWMs to
supply the current reference voltages.
The L6208PD’s H-bridges can be
disabled by applying a logical low level
to the R2-C4 RC pair. Recall that the RC
time constant provided by R2 and C4
provides a recovery off-time in the
event of an overcurrent condition.
While we have off-time on our
minds, the RC pairs of R7-C7 and R8-C8
determine the off-time of the PWM
current controller monostable. There’s
no science behind the inclusion of the
R7 and R8 resistors as potentiometers.
I placed 100K potentiometers in the R7
and R8 positions to allow for easy
tinkering. I found that I could play a
part in controlling the motor current
and thus its operation by adjusting the
off-time of the PWM current controller
monostable using a screwdriver.
With the layout for the L6208PD and
its band of brothers down on fiberglass,
let’s turn our attention to planting some
pads for the PIC. Obviously, applying the
maximum VSD voltage to the PIC VDD
pin will allow the Microchip magic smoke
to escape. So, we’ll need to design in a
voltage regulator system that can
withstand the 24 VDC maximum bulk
voltage at its input and provide + 5. 12
VDC to the VDD pin of our PIC18F2620.
Since there are no fixed linear
voltage regulators that will produce
exactly + 5. 12 VDC at their outputs,
we’ll resort to designing in a variable
voltage regulator. I have chosen
the low-dropout first cousin of the
venerable LM317, the LM1084-ADJ.
If you’re wondering why I say we
need to have + 5. 12 VDC on the
PIC18F2620’s VDD pin, the answer is
“analog-to-digital converter.” We won’t
be employing the services of the
PIC18F2620’s 10-bit analog-to-digital
converter subsystem in this project.
However, there is a possibility that you
may want to use the PIC18F2620’s A-to-D converter in your L6208PD project.
We’re using the LM1084-ADJ because it
regulates effectively with input/output
voltage differentials of up to 29V. Sure,
we could have used the fixed LM1084-
5.0 to give us a nominal + 5 VDC for the
PIC as it has a 25V input/output voltage
differential. However, saving that pair of
sub-penny resistors is a solution I
refused in favor of a more accurate A-to-D converter reference voltage.
Using the PIC18F2620’s VDD input
voltage as the A-to-D converter
reference voltage yields a 5 mV per
step resolution on your A-to-D converter
readings when VDD is equal to + 5. 12
volts. Regardless of which LM1084
logic power supply solution you choose
to use in your project, you can’t exceed
35V at the VSD input with our LM1084
power supply design. If you exceed 30V
at the VSD input with the LM1084-5.0
or 35V at the VSD input with the
LM1084-ADJ, you’ll have to supply the
+ 5 VDC logic power in another way.
What you see on the fiberglass in
Screenshot 3 and in the power supply
section of the L6208PD Motor Driver
Board schematic works well for this particular spin of the L6208PD electronics.
Schematic 1 shows a “variable”
value for the voltage adjust resistor R11.
