LTC3621/LTC3621-2

7

3621f

For more information www.linear.com/LTC3621

operaTion

The LTC3621 uses a constant-frequency, peak current

and regulates with ultralow quiescent current. The opera-

tion frequency is set at either 2.25MHz or 1MHz. To suit

a variety of applications, the selectable MODE pin allows

the user to trade off output ripple for efficiency.

The output voltage is set by an external divider returned to

the FB pin. An error amplifier compares the divided output

voltage with a reference voltage of 0.6V and adjusts the

peak inductor current accordingly. In the MS8E package,

overvoltage and undervoltage comparators will pull the

PGOOD output low if the output voltage is not within 7.5%

of the programmed value. The PGOOD output will go high

immediately after achieving regulation and will go low 32

clock cycles after falling out of regulation.

Main Control Loop

During normal operation, the top power switch (P-channel

MOSFET) is turned on at the beginning of a clock cycle.

The inductor current is allowed to ramp up to a peak level.

Once that level is reached, the top power switch is turned

off and the bottom switch (N-channel MOSFET) is turned

on until the next clock cycle. The peak current level is con-

trolled by the internally compensated ITH voltage, which is

the output of the error amplifier. This amplifier compares

the FB voltage to the 0.6V internal reference. When the

load current increases, the FB voltage decreases slightly

below the reference, which causes the error amplifier to

increase the ITH voltage until the average inductor current

matches the new load current.

The main control loop is shut down by pulling the RUN

pin to ground.

Low Current Operation

Twodiscontinuous-conductionmodes(DCMs)areavailable

to control the operation of the LTC3621 at low currents.

Both modes, Burst Mode operation and pulse-skipping,

automatically switch from continuous operation to the

selected mode when the load current is low.

To optimize efficiency, Burst Mode operation can be se-

operation, the peak inductor current is set to be at least

400mA, even if the output of the error amplifier demands

less.Thus,whentheswitcherisonatrelativelylightoutput

loads, FB voltage will rise and cause the ITH voltage to

drop. Once the ITH voltage goes below 0.2V, the switcher

goes into its sleep mode with both power switches off.

The switcher remains in this sleep state until the external

load pulls the output voltage below its regulation point.

During sleep mode, the part draws an ultralow 3.5µA of

selected by grounding the MODE pin. In the LTC3621,

pulse-skipping mode is implemented similarly to Burst

Mode operation with the peak inductor current set to be

at about 66mA. This results in lower output voltage ripple

than in Burst Mode operation with the trade-off being

slightly lower efficiency.

Forced Continuous Mode Operation

Aside from the two discontinuous-conduction modes,

the LTC3621 also has the ability to operate in the forced

continuous mode by setting the MODE voltage between

switcher will switch cycle by cycle regardless of what

the output load current is. If forced continuous mode is

selected, the minimum peak current is set to be –133mA

in order to ensure that the part can operate continuously

at zero output load.

High Duty Cycle/Dropout Operation

Whentheinputsupplyvoltagedecreasestowardstheoutput

voltage, the duty cycle increases and slope compensation

is required to maintain the fixed switching frequency. The

LTC3621 has internal circuitry to accurately maintain the

As the duty cycle approaches 100%, the LTC3621 enters

dropout operation. During dropout, if force continuous

mode is selected, the top PMOS switch is turned on

continuously, and all active circuitry is kept alive. How-

ever, if Burst Mode operation or pulse-skipping mode is

selected, the part will transition in and out of sleep mode

depending on the output load current. This significantly

reduces the quiescent current, thus prolonging the use

of the input supply.