SI9961A Ver la hoja de datos (PDF) - Vishay Semiconductors

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SI9961A

12-V Voice Coil Motor Driver

Vishay Semiconductors 

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 compensa-

tion 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-con-

duction 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 > > 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 volt-

age 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 consider-

ation, 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 accept-

able. One must weigh the servo bandwidth, phase margin

degradation, and maximum voltage at the VCM for each indi-

vidual case.

Si9961A

Vishay Siliconix

Result:

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

shoot and proper pole cancellation, the compensation values

are:

RL = 6.2 kΩ

CL = 0.016 µF

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

shoot and proper pole cancellation, the compensation values

are:

RL = 4.7 kΩ

CL = 0.022 µF

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 Ω . Any mismatch between RSA and RSB

contribute directly to mismatch between the positive and

negative "full-scale". Including the external resistor mis-

match, 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 Ω "5 %

- 2 Rm = 52 Ω

-3

Gm = 500 mA/V

-4

-5

- 300 - 200 - 100 0

100 200 300

VIN in mV

Figure 2. Si9961A Transconductance

End Point Non-Linearity

Document Number: 70014

S-40845-Rev. H, 03-May-04

www.vishay.com

7

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