TDA1085C

4

MOTOROLA ANALOG IC DEVICE DATA

GENERAL DESCRIPTION

The TDA 1085C triggers a triac accordingly to the speed regulation

requirements. Motor speed is digitally sensed by a tachogenerator

and then converted into an analog voltage.

The speed set is externally fixed and is applied to the internal linear

regulation input after having been submitted to programmable

acceleration ramps. The overall result consists in a full motor speed

range with two acceleration ramps which allow efficient washing

machine control (Distribute function).

Additionally, the TDA 1085C protects the whole system against AC

line stop or variations, overcurrent in the motor and tachogenerator

failure.

INPUT/OUTPUT FUNCTIONS

(Refer to Figures 1 and 8)

Voltage Regulator – (Pins 9 and 10) This is a parallel type regulator

able to sink a large amount of current and offering good

characteristics. Current flow is provided from AC line by external

dropping resistors R1, R2, and rectifier: This half wave current is

used to feed a smoothering capacitor, the voltage of which is

checked by the IC.

When VCC is reached, the excess of current is derived by another

dropping resistor R10 and by Pin 10. These three resistors must be

determined in order:

CC

consumption is maximum (fast ramps and pulses present).

10 reach 3.0 V when AC line provides maximum current and

VCC consumption is minimum (no ramps and no pulses).

exceeded unless loss of regulation.

An AC line supply failure would cause shut down.

The double capacitive filter built with R1 and R2 gives an efficient

VCC smoothing and helps to remove noise from set speeds.

Speed Sensing – (Pins 4, 11, 12) The IC is compatible with an

external analog speed sensing: its output must be applied to Pin 4,

and Pin 12 connected to Pin 8.

In most of the applications it is more convenient to use a digital

speed sensing with an unexpensive tachogenerator which

doesn

′t need any tuning. During every positive cycle at Pin 12,

the capacitor CPin 11 is charged to almost VCC and during this

time, Pin 4 delivers a current which is 10 times the one charging

CPin 11. The current source gain is called G and is tightly

specified, but nevertheless requires an adjustment on RPin 4. The

current into this resistor is proportional to CPin 11 and to the motor

speed; being filtered by a capacitor, VPin 4 becomes smothered

and represents the “true actual motor speed”.

To maintain linearity into the high speed range, it is important to verify

that CPin 11 is fully charged: the internal source on Pin 11 has 100 KΩ

impedance. Nevertheless CPin 11 has to be as high as possible as it

has a large influence on FV/C temperature factor. A 470 K

Ω resistor

between Pins 11 and 9 reduces leakage currents and temperature

factor as well, down to neglectable effects.

Pin 12 also has a monitoring function: when its voltage is above

5.0 V, the trigger pulses are inhibited and the IC is reset. It also

senses the tachogenerator continuity, and in case of any circuit

aperture, it inhibits pulse, avoiding the motor to run out of control. In

the TDA 1085C, Pin 12 is negatively clamped by an internal diode

which removes the necessity of the external one used in the former

circuit.

Ramp Generator – (Pins 5, 6, 7) The true Set Speed value taken in

consideration by the regulation is the output of the ramp generator

(Pin 7). With a given value of speed set input (Pin 5), the ramp

generator charges an external capacitor CPin 7 up to the moment

VPin 5 (set speed) equals VPin 4 (true speed), see Figure 2. The IC

has an internal charging current source of 1.2mA and delivers it from

0 to 12 V at Pin 7. It is the high acceleration ramp (5.0 s typical) which

allows rapid motor speed changes without excessive strains on the

mechanics. In addition, the TDA 1085C offers the possibility to break

this high acceleration with the introduction of a low acceleration ramp

(called Distribution) by reducing the Pin 7 source current down to

5.0

µA under Pin 6 full control, as shown by following conditions:

by VPin 6 x VPin 4 x 2.0 VPin 6

For two fixed values of VPin 5 and VPin 6, the motor speed will have

high acceleration, excluding the time for VPin 4 to go from VPin 6

to two times this value, high acceleration again, up to the moment

the motor has reached the set speed value, at which it will stay,

see Figure 3.

Should a reset happen (whatever the cause would be), the above

mentioned successive ramps will be fully reprocessed from 0 to the

maximum speed. If VPin 6 = 0, only the high acceleration ramp

occurs.

To get a real zero speed position, Pin 5 has been designed in such a

way that its voltage from 0 to 80 mV is interpreted as a true zero. As

a consequence, when changing the speed set position, the designer

must be sure that any transient zero would not occur: if any, the entire

circuit will be reset.

As the voltages applied by Pins 5 and 6 are derived from the internal

voltage regulator supply and Pin 4 voltage is also derived from the

same source, motor speed (which is determined by the ratios

between above mentioned voltages) is totally independent from VCC

variations and temperature factor.

Control Amplifier – (Pin 16) It amplifies the difference between true

speed (Pin 4) and set speed (Pin 5), through the ramp generator. Its

output available at Pin 16 is a double sense current source with a

maximum capability of

± 100 µA and a specified transconductance

(340

µA/V typical). Pin 16 drives directly the trigger pulse generator,

and must be loaded by an electrical network which compensates the

mechanical characteristics of the motor and its load, in order to

provide stability in any condition and shortest transient response; see

Figure 4.

This network must be adjusted experimentally.

In case of a periodic torque variations, Pin 16 directly provides the

phase angle oscillations.