LT3470A

9

3470afa

Figure 1. Operating Waveforms of the LT3470A Converting 12V to 5V Using a 33μH Inductor and 10μF Output Capacitor

OPERATION

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

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).

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

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

typically 100nA.

(1a) Burst Mode Operation

(1b) Continuous Operation

20mV/DIV

100mA/DIV

1ms/DIV

20mV/DIV

100mA/DIV

5μs/DIV

3470a F01a

NO LOAD

10mA LOAD

20mV/DIV

100mA/DIV

1μs/DIV

20mV/DIV

100mA/DIV

1μs/DIV

3470a F1b

200mA LOAD

150mA LOAD