LTM4656 datasheet(15/28 Pages) AD

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LTM4656/LTM4656-1

Rev. 0

For more information www.analog.com

Light Load Current Operation—Burst Mode Operation,

Pulse-Skipping or Continuous Conduction

(MODE_PLLIN Pin)

The LTM4656 boost converter can be enabled to enter

high efficiency Burst Mode operation, constant-frequency

pulse-skipping mode or forced continuous conduction

mode at low load currents. To select Burst Mode opera-

tion, tie the MODE_PLLIN pin to ground. To select forced

continuous operation, tie the MODE_PLLIN pin to INTVCC.

To select pulse-skipping mode, tie the MODE_PLLIN pin to

a DC voltage greater than 1.2V and less than INTVCC–1.3V.

When the controller is enabled for Burst Mode operation,

the minimum peak current in the inductor is set to approxi-

mately 30% of the maximum sense voltage even though

the voltage on the COMP pin indicates a lower value.

If the average inductor current is higher than the required

current,theinternalerroramplifierwilldecreasethevoltage

on the COMP pin. When the COMP voltage drops below

0.425V, an internal sleep signal goes high (enabling sleep

mode) and both external MOSFETs are turned off. The

COMP pin is then disconnected from the output of the

EA and parked at 0.450V.

In sleep mode, much of the internal boost controller

circuitry is turned off and the LTM4656 draws only 50µA

of quiescent current. In sleep mode, the load current is

supplied by the output capacitor. As the output voltage

decreases,theinternalerroramplifieroutputbeginstorise.

When the output voltage drops enough, the COMP pin is

reconnectedtotheoutputoftheinternalerroramplifier,the

sleep signal goes low, and the controller resumes normal

operation by turning on the bottom external MOSFET on

the next cycle of the internal oscillator. When the controller

is enabled for Burst Mode operation, the inductor current

is not allowed to reverse. The internal reverse current

comparator (IR) turns off the top external MOSFET just

before the inductor current reaches zero, preventing it

from reversing and going negative. Thus, the controller

operates in discontinuous current operation.

APPLICATIONS INFORMATION

In forced continuous operation or when clocked by an

external clock source to use the phase-locked loop. See

the Frequency Selection and Phase-Locked Loop (FREQ

and MODE_PLLIN Pins) section, the inductor current is

allowed to reverse at light loads or under large transient

conditions. The peak inductor current is determined by

the voltage on the COMP pin, just as in normal operation.

In this mode, the efficiency at light loads is lower than in

BurstModeoperation.However,continuousoperationhas

the advantages of lower output voltage ripple and less

interference to audio circuitry, as it maintains constant-

frequency operation independent of load current.

When the MODE_PLLIN pin is connected for pulse-

skipping mode, the LTM4656 boost converter operates

in PWM pulse-skipping mode at light loads. In this mode,

constant-frequency operation is maintained down to ap-

proximately 1% of designed maximum output current.

At very light loads, the internal current comparator may

remain tripped for several cycles and force the external

bottom MOSFET to stay off for the same number of cycles

(i.e., skipping pulses). The inductor current is not allowed

to reverse (discontinuous operation). This mode, like

forced continuous operation, exhibits low output ripple

as well as low audio noise and reduced RF interference

as compared to Burst Mode operation. It provides higher

low current efficiency than forced continuous mode, but

not nearly as high as Burst Mode operation.

Frequency Selection and Phase-Locked Loop (FREQ

and MODE_PLLIN Pins)

Theselectionofswitchingfrequencyisatrade-offbetween

efficiency and component size. Low frequency operation

increasesefficiencybyreducingMOSFETswitchinglosses,

butrequireslargerinductanceand/orcapacitancetomain-

tain low output ripple voltage. The switching frequency

of the LTM4656 boost controllers can be selected using

the FREQ pin. If the MODE_PLLIN pin is not being driven

by an external clock source, the FREQ pin can be tied to

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