U211B datasheet(4/20 Pages) TEMIC

4 / 20 page

U211B2/ B3

TELEFUNKEN Semiconductors

Rev. A1, 29-May-96

4 (20)

VC3

V12

ttot

95 10272

Figure 4. Soft-start

= build-up of supply voltage

= charging of C3 to starting voltage

t1 + t2 = dead time

= run-up time

ttot

= total start-up time to required speed

C3 is first charged up to the starting voltage V0 with

typical 45

mA current (t2). By then reducing the charging

current to approx. 4

mA, the slope of the charging function

is substantially reduced so that the rotational speed of the

motor only slowly increases. The charging current then

increases as the voltage across C3 increases giving a

progressively rising charging function which accelerates

the motor more and more strongly with increasing

rotational speed. The charging function determines the

acceleration up to the set-point. The charging current can

have a maximum value of 55

mA.

Frequency to Voltage Converter

The internal frequency to voltage converter (f/V-

converter) generates a DC signal on Pin 10 which is

proportional to the rotational speed using an AC signal

from a tacho-generator or a light beam whose frequency

is in turn dependent on the rotational speed. The high

impedance input Pin 8, compares the tacho-voltage to a

switch-on threshold of typ. –100 mV. The switch-off

threshold is given with –50 mV.

The

hysteresis

guarantees very reliable operation even when relatively

simple tacho-generators are used. The tacho-frequency is

given by:

+ n

p(Hz)

where:

n = revolutions per minute

p = number of pulses per revolution

The converter is based on the charge pumping principle.

With each negative half wave of the input signal, a

quantity of charge determined by C5 is internally

amplified and then integrated by C6 at the converter

output on Pin 10. The conversion constant is determined

by C5, its charge transfer voltage of Vch, R6 (Pin 10) and

the internally adjusted charge transfer gain.

k = Gi

Vch

+ 8.3

The analog output voltage is given by

VO = k

@ f

The values of C5 and C6 must be such that for the highest

possible input frequency, the maximum output voltage

VO does not exceed 6 V. While C5 is charging up, the Ri

on Pin 9 is .approx. 6.7 k

W. To obtain good linearity of the

f/V converter the time constant resulting from Ri and C5

should be considerably less (1/5) than the time span of the

negative half-cycle for the highest possible input

frequency. The amount of remaining ripple on the output

voltage on Pin 10 is dependent on C5, C6 and the internal

charge amplification.

∆VO =

Vch

The ripple

∆Vo can be reduced by using larger values of

C6. However, the increasing speed will then also be

reduced.

The value of this capacitor should be chosen to fit the

particular control loop where it is going to be used.

Pulse Blocking

The output of pulses can be blocked using Pin 18 (standby

operation) and the system reset via the voltage monitor if

V18 ≥ –1.25 V. After cycling through the switching point

hysteresis, the output is released when V18 ≤ –1.5 V

followed by a soft-start such as that after turn on.

Monitoring of the rotation can be carried out by

connecting an RC network to Pin 18. In the event of a

short or open circuit, the triac triggering pulses are cut off

by the time delay which is determined by R and C. The

capacitor C is discharged via an internal resistance

Ri = 2 k

W with each charge transfer process of the f/V

converter. If there are no more charge transfer processes

C is charged up via R until the switch-off threshold is

exceeded and the triac triggering pulses are cut off. For

operation without trigger pulse blocking or monitoring of

the rotation, Pins 18 and 16 must be connected together.

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