TLE 5250

Semiconductor Group

6

1998-02-01

Nominal-Current Input

The peak current in the motor winding is defined by the voltage on the Nominal input.

This is compared by a fast comparator to the voltage drop on the actual-current sensor.

If the nominal current is exceeded, the sink transistors of the outputs are turned off by

the logic.

RC/Sync Input

The outputs are turned on by the signal applied to the RC input. Synchronization is

possible by TTL signal or chopper mode with an external RC combination.

Chopper Mode

After the supply voltage is applied, capacitor CT is charged with constant current of

1 mA. A regulator limits the maximum voltage on the capacitor to 2.3 V. As a result of the

rising current in the motor winding, the voltage on the actual sensor increases. Once the

value defined by the nominal-current input is exceeded, the fast comparator resets an

RS flipflop. Thus sink transistors T3 and T4 are turned off by the logic. The charge

current is turned off and the parallel RT discharges CT.

The internal logic is designed so that capacitor CT is always charged before the

discharge operation is triggered. This guarantees a constant charge time, even for very

small coil currents (see Figure 7).

Sync Operation

If a sync signal with TTL level is applied to the RC input, the negative edge will set the

RS flipflop - by way of the combined Schmitt trigger and monoflop - if the voltage on the

current sensor is smaller than the nominal value on the nominal-current input. As in

chopper mode, the appropriate output transistors conduct. They are again turned off by

resetting the RS flipflop when the voltage on the current sensor becomes greater than

the nominal value (see Figure 8).

Output-Stage Control

This part of the circuit handles turn-off of the output stages when the output is shorted to

ground. There is separate current monitoring for this purpose in the source transistors.

The temperature of the output stages is also monitored. If this exceeds 175

°C, all output

stages are turned off, and then turned on again when the temperature drops.

Undervoltage also causes turn-off of the transistors in the output stages. These possible

fault states are stored in the diagnostics register.