AN458 APPLICATION NOTE

10/19

When we are operating in self-oscillating mode, the comparator that senses the free-wheeling status disables

the oscillator pulses output until the inductor is fully discharged, varying in this way the switching frequency. It

is also possible to disable the oscillator forcing the system to operate always in self-oscillating mode, connec-

tingtogether the internal oscillator capacitor (pin.17) with the voltage reference pin.11.

Figure 12. Oscillator Circuit

CURRENT LIMITATION

Output overload protection is provided by a current limiter circuit. The load current is sensed by an internal metal

resistor (Rs) in series to the power transistor. When the voltage drop on the sense resistor, reaches the current

comparator offset voltage, the current comparator generates a reset pulse for the latch, disabling the power

stage.

Typical current limiting threshold is around 4.5A. The power stage will be enabled again only when the energy

stored in the inductor will be completely discharged, this due to the free-wheeling sense comparator (see Circuit

operation paragraph for details).

The current limiting circuit operates also as softstart during the device turn-on preventing overcurrents on the

load. In fig.13 is shown the simplified internal schematic circuit of the current limiter.

RESET

The reset circuit accomplishes a very important function when the L4963 is used in applications where it feeds

microprocessors and logic devices. The function block diagram is shown in Fig.14. The Reset circuit monitors

the output voltage and generates a logic signal when the output voltage is within the limits required to supply

correctly the microprocessor.

This function is realized through three pins:

s

Feedback input (pin.12)

s

Reset delay (pin.9)

s

Reset output (pin.10)

When the monitored voltage on pin.12 is lower than 5V, the comparator (A) output is high and the reset delay

capacitor is not charged because the transistor Q1 is saturated, also the transistor Q2 is saturated, mantaining

the voltage on pin.10 at low level.