BUS VOLTAGE FILTER CAPACITORS

The size and placement of the capacitors for the DC bus has a direct bearing on the amount of noise filtered and also on the size and

duration of the voltage spikes seen by the bridge. What is being created is a series RLC tuned circuit with a resonant frequency that is

seen as a damped ringing every time one of the transistors switches. For the resistance, wire resistance, power supply impedance and

capacitor ESR all add up for the equivalent lumped resistance in the circuit. The inductance can be figured at about 30 nH per inch from

the power supply. Any voltage spikes are on top of the bus voltage and the back EMF from the motor. All this must be taken into account

when designing and laying out the system. If everything has been minimized, there is another solution. A second capacitance between

5 and 10 times the first capacitor and it should either have some ESR or a resistor can be added in series with the second capacitor to

help damp the voltage spikes.

Be careful of the ripple current in all the capacitors. Excessive ripple current, beyond what the capacitors can handle, will destroy the

capacitors.

INPUT VOLTAGE FILTER CAPACITORS

It is recommended that about 10 μF of capacitance (tantalum electrolytic) for bypassing the + and -15V power supplies be placed as

close to the module pins as practical. Adding ceramic bypass capacitors of about 0.1 μF to 1 μF will aid in suppressing noise transients.

GENERAL LAYOUT

Good PC layout techniques are a must. Ground plane for the analog circuitry must be used and should be tied back to the SIG GND.

Ground plane for the power circuitry should be tied back to the GND pin, pin 16. Pin 16 should be connected to pin 11 external to the

hybrid by a single thick trace. This will connect the two ground planes together.

LOW POWER STARTUP

When starting up a system utilizing the MSK4310 for the first time, there are a few things to keep in mind. First, because of the small size

of the module, short circuiting the output phases either to ground or the DC bus will destroy the bridge. The current limiting and control

only works for current actually flowing through the bridge. The current sense resistor has to see the current in order for the electronics

to control it. If possible, for startup use a lower voltage and lower current power supply to test out connections and the low current

stability. With a limited current supply, even if the controller locks up, the dissipation will be limited. The ±15 volt power should be applied

prior to applying the V+ voltage.

5

APPLICATION NOTES CONTINUED

8548-72 Rev. F 12/14