L6238S datasheet(18/31 Pages) STMICROELECTRONICS

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a voltage, Vbrake, during brake mode.

The maximum capacitance specified for the Stor-

age Capacitor is 4.7

µF.For applications requiring

a larger value, an external diode should be con-

nected between Vanalog and the

Storage Ca-

pacitor to prevent excessive inrush current from

damaging the charge pump circuitry. A small

value resistor (i.e. 50W) may instead be inserted

in series with the Storage Capacitor to limit the in-

rush current.

4.4 Linear Motor Current Control

The output current is controlled in a linear fashion

via a transconductance loop. Referring to Figure

4-4 the sourcing FET of one phase is forced into

full conduction by connecting the gate to Vpump,

while the sinking transistor of an appropriate

phase operates as a transconductance element.

To understand the current control loop, it will be

assumed that Q2 in figure 4-4 is enabled via SW3

by the sequencer. During a run condition, the cur-

rent in Q2 is monitored by a resistor Rs connected

to the Rsense input.

The resulting voltage that appears across Rs is

amplified by a factor of four by A3 and is sent to

A2 where it is compared to the Current Command

Signal. A2 provides sufficient drive to Q2 in order

to maintain the motor speed at the proper level as

commanded by the Speed Controller.

4.5 Transconductance Loop Stability

The RC network connected to the Compensation

pin provides for a single pole/zero compensation

scheme. The pole/zero compensation scheme.

The pole/zero locations are adjusted such that a

few dB of gain (typ. 20dB) remains in the tran-

sconductance loop at frequencies higher than the

zero.

The inductive characteristic of the load provides

the pole necessary for loop stability. Thus the

loop bandwidth is actually limited by the motor it-

self.

Figure 4-5 shows the complete transconductance

loop including compensation, plus the response.

The Bode plot depicts the normal way to achieve

stability in the loop. The pole andzero are used to

set a gain of 20dB at a higher frequency and the

pole of the motor cuts the gain to achieve stabil-

ity.

Loop instability may be caused by two factors:

1)The motor pole is too close to the zero. Refer-

ring to figure 4.6, the zero is not able to dec-

rement the shift of phase, and when the effect

of the pole is present, the phase shift may

reach 180

° and the loop will oscillate. To rec-

tify this situation, the pole/zero must be

shifted at lower frequencies by increasing the

compensation capacitor.

Figure 4-5

Figure 4-6

Figure 4-7

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