LT3470AEDDB#TRMPBF Linear Technology, LT3470AEDDB#TRMPBF Datasheet - Page 8

• Manufacturer Part Number: LT3470AEDDB#TRMPBF
• Description: IC BUCK ADJ .25A 8DFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3470AEDDB#TRMPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 16 V
• Current - Output: 250mA
• Voltage - Input: 4 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-DFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Frequency - Switching: -

OPERATION
LT3470A
The LT3470A 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 inductor 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
shut off all the high power circuitry, leaving the LT3470A
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).
two current comparators as follows: The switch turns on,
and the current through it ramps up until the top current
8
The switching action is controlled by an RS latch and
100mA/DIV
100mA/DIV
20mV/DIV
20mV/DIV
Figure 1. Operating Waveforms of the LT3470A Converting 12V to 5V Using a 33μH Inductor and 10μF Output Capacitor
V
V
OUT
OUT
I
I
L
L
NO LOAD
10mA LOAD
(1a) Burst Mode Operation
1ms/DIV
5μs/DIV
REF
, the error amplifier will
3470a F01a
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 bot-
tom 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
typically 100nA.
100mA/DIV
100mA/DIV
20mV/DIV
20mV/DIV
V
V
IN
OUT
OUT
I
I
L
L
current reduces to the device leakage current,
200mA LOAD
150mA LOAD
(1b) Continuous Operation
1μs/DIV
1μs/DIV
3470a F1b
3470afb