LT1934-1 Linear Technology, LT1934-1 Datasheet - Page 6

• Manufacturer Part Number: LT1934-1
• Description: Micropower Step-Down Switching Regulators in ThinSOT
• Manufacturer: Linear Technology

OPERATIO
LT1934/LT1934-1
The LT1934 uses Burst Mode control, combining both low
quiescent current operation and high switching frequency,
which result in high efficiency across a wide range of load
currents and a small total circuit size.
A comparator monitors the voltage at the FB pin of the
LT1934. If this voltage is higher than the internal 1.25V
reference, the comparator disables the oscillator and power
switch. In this state, only the comparator, reference and
undervoltage lockout circuits are active, and the current
into the V
the output capacitor, the voltage at the FB pin falls below
1.25V and the comparator enables the oscillator. The
LT1934 begins to switch, delivering current to the output
capacitor. The output voltage rises, and when it overcomes
the feedback comparator’s hysteresis, the oscillator is
disabled and the LT1934 returns to its micropower state.
The oscillator consists of two one-shots and a flip-flop.
A rising edge from the off-time one-shot sets the flip-
flop, which turns on the internal NPN power switch. The
switch remains on until either the on-time one-shot trips
or the current limit is reached. A sense resistor and
amplifier monitor the current through the switch and resets
6
IN
pin is just 12 A. As the load current discharges
U
(Refer to Block Diagram)
50mV/DIV
0.5A/DIV
0.5A/DIV
10V/DIV
Figure 1. Operating Waveforms of the LT1934 Converting
10V to 3.3V at 180mA (Front Page Schematic)
V
V
I
OUT
SW
SW
I
LI
5 s/DIV
the flip-flop when this current reaches 400mA (120mA
for the LT1934-1). After the 1.8 s delay of the off-time
one-shot, the cycle repeats. Generally, the LT1934 will
reach current limit on every cycle—the off time is fixed
and the on time is regulated so that the LT1934 operates
at the correct duty cycle. The 1.8 s off time is lengthened
when the FB pin voltage falls below 0.8V; this foldback
behavior helps control the output current during start-up
and overload. Figure 1 shows several waveforms of an
LT1934 producing 3.3V from a 10V input. When the switch
is on, the SW pin voltage is at 10V. When the switch is off,
the inductor current pulls the SW pin down until it is
clamped near ground by the external catch diode.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate the
bipolar switch for efficient operation.
If the SHDN pin is grounded, all internal circuits are turned
off and V
typically a few nA.
IN
current reduces to the device leakage current,
1934 F01a
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