4

4778B–AUTO–09/05

ATA6821

3.

Functional Description

The ATA6821 controls a push-pull power output stage for an external power device and a feed-

back output stage. If the voltage at the input pin is > 7V, both outputs are at “high-level”, if the

input voltage falls below the switch-off threshold of 4V, both outputs are switched to ground.

If a rectangular input signal is fed to the input, the feedback output “Feed” is not used and pin 8

needs no external connection.

In case the input signal, however, comes from a transformer (potential-free control; see Figure

7-5 on page 12), the integrated pulse shaping circuit is used to achieve a reliable feedback con-

figuration at the input. This ensures correct switching of the output stage even if the input signal

is very noisy.

For the timing of input and output signals and the slew rates, please refer to Figure 7-1 on page

9. The timing diagram is based on the following test conditions: A 2 k

Ω resistor is connected

between pin 8 and ground; a 2 k

Ω resistor and a 1 nF capacitor are used as load for the power

output (pins 13 and 14) simulating identical conditions as a power MOSFET, which is usually

connected to the output.

3.1

Undervoltage Detection

During power-up and power-down of the supply voltage, the internal undervoltage circuit sup-

presses uncontrolled output pulses. In case of undervoltage (V

the power output are switched off and locked for 40 ms.

When the supply voltage has reached the switch-on-threshold of about 17V, both output stages

are enabled after a delay time of 40 ms.

3.2

Voltage Monitoring

An effective short-circuit detection of the external device is achieved by a drain-source voltage

monitor.

The voltage monitor comparator (pin 3) has a threshold of 1/3

× V

3 exceeds this voltage, both output stages are switched off immediately and locked for 20 ms.

With a hysteresis of 200 mV, the outputs are enabled again after a delay time of 20 ms. A volt-

age divider connected to V

recommended if the external power device‘s drain source voltage exceeds 30V (see Figure 7-3

on page 10).

With the help of an external capacitor, connected to pin 3, the switching characteristic of the volt-

age monitor comparator can be varied according to the rising edge of the input signal at pin 9.

During the off cycle, the capacitor is discharged, during the on-cycle, it is charged again.

If the voltage monitoring function is not used, pin 3 should be connected to ground (see Figure

7-2 on page 10).