TDA 4916 GG

Semiconductor Group

5

05.96

Circuit Description

The individual functional sections of the device and their interactions are described

below.

Power Supply at

V

S

The device does not enable the output until the turn-ON threshold of

V

duty factor (active time/period) can then rise from zero to the value set with K1 in the time

determined by the soft start. The turn-OFF threshold lies below the turn-ON threshold.

Below the turn-OFF threshold the output Q SIP is reliably low.

Frequency Generator

The frequency is mainly determined by close-tolerance external components and the

calibrated reference voltage.

The switching frequency at the output can be set by suitable choice of

R

The maximum possible duty factor can be reduced by a defined amount by means of a

resistor from

C

defined amount by means of a resistor from

C

Ramp Generator

The ramp generator is controlled by the frequency generator and operates with the same

frequency. Capacitor

C

and charged via a current set externally. The duration of the falling edge of the ramp

generator output must be shorter than its rise time. Only then do the upper and lower

switching levels of the ramp generator signal have their nominal values.

In “voltage mode control” operation, the rising edge of the ramp generator signal is

compared with an externally set dc voltage in comparator K1 for pulse-width control at

the output. The slope of the rising edge is set by the current through

R

source connected to

R

the duty factor for a constant volt-second product at the output. This control option

(precontrol) permits equalization of known disturbances (e.g. input voltage ripple).

Superimposed load current control (current mode control) can also be implemented. For

this purpose the actual current at the source of the SIPMOS transistor is sensed and

compared with the specified value in comparator K5.