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LT3470ETS8 Datasheet

  • LT3470ETS8

  • Micropower Buck Regulator with Integrated Boost and Catch Di...

  • 16頁(yè)

  • Linear

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LT3470
OPERATIO
The LT3470 uses a hysteretic control scheme in conjunc-
tion with Burst Mode operation to provide low output
ripple and low quiescent current while using a tiny induc-
tor and capacitors.
Operation can best be understood by studying the Block
Diagram. An error amplifier measures the output voltage
through an external resistor divider tied to the FB pin. If
the FB voltage is higher than V
REF
, the error amplifier will
shut off all the high power circuitry, leaving the LT3470 in
its micropower state. As the FB voltage falls, the error
amplifier will enable the power section, causing the chip
to begin switching, thus delivering charge to the output
capacitor. If the load is light the part will alternate between
micropower and switching states to keep the output in
regulation (See Figure 1a). At higher loads the part will
switch continuously while the error amp servos the top
and bottom current limits to regulate the FB pin voltage to
1.25V (See Figure 1b).
The switching action is controlled by an RS latch and two
current comparators as follows: The switch turns on, and
the current through it ramps up until the top current
NO LOAD
V
OUT
20mV/DIV
V
OUT
20mV/DIV
I
L
100mA/DIV
I
L
100mA/DIV
1ms/DIV
10mA LOAD
V
OUT
20mV/DIV
V
OUT
20mV/DIV
I
L
100mA/DIV
5碌s/DIV
3470 F01a
(1a) Burst Mode Operation
Figure 1. Operating Waveforms of the LT3470 Converting 12V to 5V Using a 33碌H Inductor and 10碌F Output Capacitor
U
comparator trips and resets the latch causing the switch to
turn off. While the switch is off, the inductor current ramps
down through the catch diode. When both the bottom
current comparator trips and the minimum off-time one-
shot expires, the latch turns the switch back on thus
completing a full cycle. The hysteretic action of this control
scheme results in a switching frequency that depends on
inductor value, input and output voltage. Since the switch
only turns on when the catch diode current falls below
threshold, the part will automatically switch slower to keep
inductor current under control during start-up or short-
circuit conditions.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and internal diode is
used to generate a voltage at the BOOST pin that is higher
than the input supply. This allows the driver to fully
saturate the internal bipolar NPN power switch for efficient
operation.
If the SHDN pin is grounded, all internal circuits are turned
off and V
IN
current reduces to the device leakage current,
typically a few nA.
200mA LOAD
1碌s/DIV
150mA LOAD
I
L
100mA/DIV
1碌s/DIV
3470 F1b
(1b) Continuous Operation
3470f
7

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