SI9961 View Datasheet(PDF) - Vishay Semiconductors

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Description

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SI9961

12-V Voice Coil Motor Driver

Vishay Semiconductors 

Si9961

Vishay Siliconix

If a unit step voltage is applied to the above transfer function

and the inverse Laplace transform is taken, the output result is:

VO + A

p ) (x * p) x e*x t

x

Where t = time

As we can see, if x = p (i.e. if the VCM pole and compensation

amplifier zero = the transconductance closed loop pole), then

Vo reduces to A. In other words, a step input results in a step

output without overshoot. If x < p then a step input results in an

increased rise time output and no overshoot. If x > p, a step

input results in a step output with an overshoot.

If this overshoot is large enough, there may be a

cross-conduction condition in the output FETs.

Let us look at the above equation at t = 0 and t >> 0, expressed

in terms of the open loop high frequency voltage gain, A.

VO + A

VO

+p

Lv

B

At t = 0

At t uu 0

In the example shown above, p = 10,000 and A = 9.8. This

means that there is some overshoot. At t = 0, the output voltage

is 9.8 V per volt of input. At some later time, it has dropped to

7.5 V per volt of input. An overshoot of 31 % is thus produced.

The maximum overshoot voltage requires careful

consideration, since it constitutes a potentially catastrophic

problem area. If we had decided to optimize for no overshoot,

A would equal 7.5, and hence the closed loop pole (A * B / Lv)

would be 10,000, which is a frequency of 1.592 kHz. This

would have resulted in a phase margin degradation of 13_ at

the 367-Hz frequency desired. This may or may not be

acceptable. One must weigh the servo bandwidth, phase

margin degradation, and maximum voltage at the VCM for

each individual case.

Result:

In the example for the 2081-Hz roll-off case with 31%

overshoot and proper pole cancellation, the compensation

values are:

RL = 6.2 kW

CL = 0.016 mF

In the example for the 1592-Hz roll-off case with no overshoot

and proper pole cancellation, the compensation values are:

RL = 4.7 kW

CL = 0.022 mF

The linearity of the transconductance amplifier (around a

center value of 500 mA/volt) is shown in Figure 2. In this case,

the output current sense resistors (RSA and RSB) were "5%

tolerance, 0.5 W . Any mismatch between RSA and RSB

contribute directly to mismatch between the positive and

negative “full-scale”. Including the external resistor mismatch,

the overall loop nonlinearity is approximately 1% maximum

over a "250-mV input voltage range.

5

4

3

2

1

0

–1 VDD = 12 V

RSA = RSB = 0.5 W ”5%

–2 Rm = 52 W

–3

Gm = 500 mA/V

–4

–5

–300 –200 –100 0 100 200 300

VIN in mV

FIGURE 2. Si9961 Transconductance

End Point Non-Linearity

Document Number: 70014

S-20883—Rev. G, 24-Jun-02

www.vishay.com

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