Servo - March 2008

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

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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.