Datasheet

CIRCUIT OPERATION (refer to the block diagram)
TheL4962 isa monolithicstepdownswitchingregu-
latorproviding outputvoltagesfrom5.1V to 40V and
delivering 1.5A.
The regulation loop consists of a sawtooth oscilla-
tor, error amplifier, comparator and the output
stage.An errorsignalis producedby comparingthe
output voltage with a precise 5.1V on-chip refer-
ence (zener zap trimmed to ± 2%).
Thiserrorsignalisthencomparedwiththesawtooth
signal to generate the fixed frequency pulse width
modulatedpulses which drive the output stage.
The gain and frequencystability of the loop can be
adjusted by an external RC network connected to
pin 11. Closing the loop directly gives an output
voltage of 5.1V. Higher voltages are obtained by
inserting a voltage divider.
Output overcurrents at switch on are prevented by
the soft start function. The error amplifier output is
initially clamped by the external capacitor C
ss
and
allowed to rise, linearly, as this capacitoris charged
by a constant current source. Output overload pro-
tection is provided in the form of a current limiter.
The load current is sensed by an internal metal
resistor connected to a comparator. When the load
current exceedsa presetthresholdthis comparator
sets a flip flop which disables the output stage and
discharges the soft start capacitor. A second com-
parator resets the flip flop when the voltage across
the soft start capacitor has fallen to 0.4V.
The output stage is thus re-enabledand theoutput
voltage rises under controlof the soft start network.
If the overload condition is still present the limiter
will trigger again when the threshold current is
reached.The average short circuit current is limited
to a safe value by the dead time introduced by the
soft start network. The thermal overload circuit dis-
ables circuit operation when the junction tempera-
ture reaches about 150°C and has hysteresis to
prevent unstable conditions.
Figure 1. Soft start waveforms
Figure 2. Current limiter waveforms
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L4962