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LTC3586/LTC3586-1

3586fb

and the input capacitance of the FBx pin and also helps

to improve transient response for output voltages much

greater than 0.8V. A variety of capacitor sizes can be used

for CFB but a value of 10pF is recommended for most ap-

plications. Experimentation with capacitor sizes between

2pF and 22pF may yield improved transient response.

Buck Regulator Operating Modes

The LTC3586/LTC3586-1’s buck regulators include two

possible operating modes to meet the noise/ power needs

of a variety of applications.

In pulse-skip mode, an internal latch is set at the start of

every cycle which turns on the main P-channel MOSFET

switch.Duringeachcycle,acurrentcomparatorcompares

thepeakinductorcurrenttotheoutputofanerrorampliﬁer.

The output of the current comparator resets the internal

latch which causes the main P-channel MOSFET switch to

turn off and the N-channel MOSFET synchronous rectiﬁer

to turn on. The N-channel MOSFET synchronous rectiﬁer

turns off at the end of the 2.25MHz cycle or if the current

through the N-channel MOSFET synchronous rectiﬁer

drops to zero. Using this method of operation, the error

ampliﬁer adjusts the peak inductor current to deliver the

required output power. All necessary compensation is

internal to the switching regulator requiring only a single

ceramic output capacitor for stability. At light loads, the

inductor current may reach zero on each pulse which will

turn off the N-channel MOSFET synchronous rectiﬁer.

In this case, the switch node (SW1, SW2) goes high

impedance and the switch node voltage will “ring”. This

is discontinuous mode operation, and is normal behavior

for a switching regulator. At very light loads, the buck

regulators will automatically skip pulses as needed to

maintain output regulation.

operaTion

At high duty cycles (VOUTx > VINx/2) it is possible for the

inductor current to reverse, causing the buck regulator

to operate continuously at light loads. This is normal and

regulationismaintained,butthesupplycurrentwillincrease

to several milliamperes due to continuous switching.

In Burst Mode operation, the buck regulator automati-

cally switches between ﬁxed frequency PWM operation

and hysteretic control as a function of the load current.

At light loads, the buck regulators operate in hysteretic

mode in which the output capacitor is charged to a volt-

age slightly higher than the regulation point. The buck

converter then goes into sleep mode, during which the

output capacitor provides the load current. In sleep mode,

most of the regulator’s circuitry is powered down, helping

conserve battery power. When the output voltage drops

below a predetermined value, the buck regulator circuitry

is powered on and the normal PWM operation resumes.

The duration for which the buck regulator operates in

sleep mode depends on the load current. The sleep time

decreases as the load current increases. Beyond a certain

loadcurrentpoint(about1/4ratedoutputloadcurrent)the

step-down switching regulators will switch to a low noise

constant frequency PWM mode of operation, much the

same as pulse-skip operation at high loads. For applica-

tions that can tolerate some output ripple at low output

currents, Burst Mode operation provides better efﬁciency

than pulse skip at light loads while still providing the full

speciﬁed output current of the buck regulator.

The buck regulators allow mode transition on the fly,

providing seamless transition between modes even under

load. This allows the user to switch back and forth between

modes to reduce output ripple or increase low current

efﬁciency as needed.

Buck Regulator in Shutdown

The buck regulators are in shutdown when not enabled for

operation. In shutdown, all circuitry in the buck regulator

is disconnected from the buck regulator input supply

leaving only a few nanoamps of leakage current. The

buck regulator outputs are individually pulled to ground

through a 10k resistor on the switch pins (SW1 and SW2)

when in shutdown.

VINx

LTC3586/

LTC3586-1

SWx

COUT

X = 1, 2

CFB

VOUTx

3586 F05

FBx

GND

Figure 5. Buck Converter Application Circuit

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