APPLICATION NOTES CONTINUED

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.

GENERAL LAYOUT

Good PC layout techniques are a must. Ground planes for the analog circuitry must be used and should be tied back to the small pin

grounds 9, 14 and 23. Additional ground, pin 26 is an isolated ground that connects internally directly back to the main DC bus ground

pin 27. This can be used as necessary for voltage sensing, etc.

LOW POWER STARTUP

When starting up a system utilizing the MSK4361 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. By observing the E/A OUT pin which is the error amp output, much

can be found out about the health and stability of the system. An even waveform with some rounded triangle wave should be observed. As

current goes up, the DC component of the waveform should move up or down. At full current (with a regular supply) the waveform should not

exceed +8 volts positive peak, or -8 volts negative peak. Some audible noise will be heard which will be the commutation frequency. If the

motor squeals, there is instability and power should be removed immediately unless power dissipation isn't excessive due to limited supply

current. For compensation calculations, refer to the block diagram for all information to determine the amplifier gain for loop gain calculations.

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8548-146 Rev. J 1/15