8

LTC3403

3403f

Main Control Loop

The LTC3403 uses a constant frequency, current mode

step-down architecture. The main (P-channel MOSFET),

synchronous (N-channel MOSFET) and bypass (P-chan-

nel MOSFET) switches are internal. During normal opera-

tion, the internal main switch is turned on each cycle when

the oscillator sets the RS latch, and turned off when the

controlled by the output of error amplifier EA. When the

load current increases, it causes a slight decrease in the

feedback voltage, FB, relative to the external reference,

which in turn, causes the EA amplifier’s output voltage to

increase until the average inductor current matches the

new load current. While the main switch is off, the syn-

chronous switch is turned on until the beginning of the

next clock cycle.

In forced continuous mode the inductor current is con-

stantly cycled. In this mode, the output voltage can re-

spond quickly to the external reference voltage by sourc-

ing or sinking current as needed.

Burst Mode Operation

The LTC3403 is capable of Burst Mode operation in which

the internal power switches operate intermittently based

on load demand.

In Burst Mode operation, the peak current of the inductor

is set to approximately 200mA regardless of the output

load. Each burst event can last from a few cycles at light

loads to almost continuously cycling with short sleep

intervals at moderate loads. In between these burst events,

the power switches and any unneeded circuitry are turned

off, reducing the quiescent current to 20

µA. In this sleep

state, the load current is being supplied solely from the

output capacitor. As the output voltage droops, the EA

amplifier’s output rises above the sleep threshold signal-

ing the BURST comparator to trip and turn the top switch

on. This process repeats at a rate that is dependent on the

load demand.

Controlling the Output Voltage

The output voltage can be dynamically programmed from

from REF is internally set to 3, the corresponding input

during operation by driving REF with an external DAC.

When REF exceeds 1.2V, an internal bypass P-channel

drop across the inductor and the main switch.

Dropout Operation

the LTC3403 enters dropout operation. During dropout,

the main switch remains on continuously and operates at

100% duty cycle. If the voltage at REF is less than 1.2V, the

bypass P-channel MOSFET will stay off even in dropout

operation. The output voltage is then determined by the

input voltage minus the voltage drop across the main switch

and the inductor.

An important detail to remember is that at low input

increases (see Typical Performance Characteristics).

Therefore, the user should calculate the power dissipa-

tion when the LTC3403 is used at 100% duty cycle with

low input voltage (See Thermal Considerations in the

Applications Information section).

Low Supply Operation

The LTC3403 will operate with input supply voltages as

low as 2.5V, but the maximum allowable output current is

reduced at this low voltage. Figure 2 shows the reduction

in the maximum output current as a function of input

voltage for various output voltages.