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.

REGULATED VOLTAGE FILTER CAPACITORS

It is recommended that about 10 µF of capacitance (tantalum electrolytic) for bypassing the + and -15V regulated outputs 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 V+ RTN pin, pin 16. Pin 16 should be connected to pin 10

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 MSK 4460 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.

5

APPLICATION NOTES CONTINUED

Rev. E 11/04