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L4964
HIGH CURRENT SWITCHING REGULATOR
4964April 1993
HIGH CURRENT SWITCHING REGULATOR 4 A OUTPUT CURRENT. 5.1 V TO 28 V OUTPUT VOLTAGE RANGE. 0 TO 100 % DUTY CYCLE RANGE. PRECISE (± 3 %) ON-CHIP REFERENCE. SWITCHING FREQUENCY UP TO 120 KHz. VERY HIGH EFFICIENCY (UP TO 90 %). VERY FEW EXTERNAL COMPONENTS. SOFT START. RESET OUTPUT. CURRENT LIMITING. INPUT FOR REMOTE INHIBIT AND SYN-
CHRONUS PWM. THERMAL SHUTDOWN
DESCRIPTIONThe L4964 is a stepdown power switching regulator
delivering 4A at a voltage variable from 5.1V to 28V.
Features of the device include overload protection,
soft start, remote inhibit, thermal protection, a reset
output for microprocessors and a PWM comparator
input for synchronization in multichip configurations.
The L4964 is mounted in a 15-lead Multiwatt plas-
tic power package and requires very few external
components.
Efficient operation at switching frequencies up to
120kHz allows a reduction in the size and cost of ex-
ternal filter components.
PIN CONNECTION (top view)1/13
PIN FUNCTIONS
BLOCK DIAGRAM
L49642/13
CIRCUIT OPERATION (refer to the block diagram)The L4964 is a monolithic stepdown switching regu-
lator providing output voltages from 5.1 V to 28 V
and delivering 4A.
The regulation loop consists of a sawtooth oscillator,
error amplifier, comparator and the output stage. An
error signal is produced by comparing the output
voltage with a precise 5.1 V on-chip reference
(zener zap trimmed to ± 3 %). This error signal is
then compared with the sawtooth signal to generate
the fixed frequency pulse width modulated pulses
which drive the output stage. The gain and fre-
quency stability of the loop can be ajusted by an ex-
ternal RC network connected to pin 9. Closing the
loop directly gives an output voltage of 5.1 V. 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 Css and al-
lowed to rise, linearly, as this capacitor is charged
by a constant current source.
Output overload protection 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 exceeds a preset threshold
this comparator sets a flip flop which disables the
output stage and discharges the soft start capacitor.
A second comparator resets the flip flop when the
voltage across the soft start capacitor has fallen to
0.4 V. The output stage is thus re-enable and the
output voltage rises under contro of the soft start net-
work. If the overload condition is still present the
limiter will trigger again when the thershold current
is reached. The average short circuit current is lim-
ited to a safe value by the dead time introduced by
the soft start network.
The reset circuit generates an output signal when
the supply voltage exceeds a threshold pro-
grammed by an external divider. The reset signal is
generated with a delay time programmed by an ex-
ternal capacitor. When the supply falls below the
threshold the reset output goes low immediately.
The reset output is an open collector.
A TTL - level input is provided for applications such
as remote on/off control. This input is activated by
high level and disables circuit operation. After an in-
hibit the L4964 restarts under control of the soft start
network.
The thermal overload circuit disables circuit opera-
tion when the junction temperature reaches about
150 and has hysteresis to prevent unstable condi-
tions.
Figure 1 : Reset Output Waveforms
L49643/13
Figure 2 : Soft Start Waveforms
Figure 3 : Current Limiter Waveforms
ABSOLUTE MAXIMUM RATINGS
THERMAL DATA
L49644/13
ELECTRICAL CHARACTERISTICS(refer to the test circuits Tj = 25o C, Vi = 25V, unless otherwise specified)
DYNAMIC CHARACTERISTICS (pin 6 to GND unless otherwise specified)
DC CHARACTERISTICS
SOFT START
INHIBIT
ERROR AMPLIFIER
L49645/13
