LTC3124 datasheet(13/28 Pages) LINER

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LTC3124

3124f

For more information www.linear.com/LTC3124

OPERATION

Boost Anti-Ringing Control

When VOUT≥VIN+2V,theanti-ringingcircuitryconnectsa

resistoracrosseachinductortoVINtodamphighfrequency

ringingontheSWpinsduringdiscontinuouscurrentmode

operation. Although the ringing of the resonant circuits

formed by the inductors and CSW(A/B) (capacitance on

the respective SW pins) is low energy, it can cause EMI

radiation if not damped.

VCC Regulator

An internal low dropout regulator generates the 4.25V

(nominal) VCC rail from VIN or VOUT, depending upon

operating conditions. VCC is supplied from VIN if VIN is

initiallygreaterthanapproximately3.5V,withVINcontinuing

to supply VCC down to approximately 3V; otherwise the

greater of VIN and VOUT supplies VCC. The VCC rail powers

theinternalcontrolcircuitryandpowerMOSFETgatedrivers

of the LTC3124. The VCC regulator is disabled in shutdown

to reduce quiescent current and is enabled by forcing the

SD pin above its input high threshold. A 4.7µF or larger

capacitor must be connected between VCC and SGND.

Overvoltage Lockout

An overvoltage condition occurs when VOUT exceeds

approximately 16.5V. Switching is disabled and the in-

ternal soft-start ramp is reset. Once VOUT drops below

approximately 16V a soft-start is initiated and switching

is allowed to resume. If the boost converter output is

lightly loaded such that the time constant of the output

capacitance, COUT, and the output load resistance, ROUT

isnearorgreaterthanthesoft-starttimeofapproximately

10ms, the soft-start ramp may end before or soon after

switchingresumes,defeatingtheinrushcurrentlimitingof

theclosed-loopsoft-startfollowinganovervoltageevent.

Short-Circuit Protection

The LTC3124 output disconnect feature allows output

short-circuit protection while maintaining a maximum set

current limit. To reduce power dissipation under overload

andshort-circuitconditions,thepeakswitchcurrentlimits

are reduced to approximately 2A. Once VOUT exceeds

approximately 1.5V, the current limits are reset to their

nominal values of 3.5A per phase.

Output Disconnect

The LTC3124’s output disconnect feature eliminates body

diode conduction of the internal P-channel MOSFET recti-

fiers.ThisfeatureallowsforVOUTtodischargeto0Vduring

shutdown, and draw no current from the input source.

Inrush current will also be limited at turn-on, minimizing

surgecurrentsseenbytheinputsupply.Notethattoobtain

the advantages of output disconnect, there must not be

an external Schottky diode connected between SWA, SWB

and VOUT. The output disconnect feature also allows VOUT

to be pulled high, without backfeeding the power source

connected to VIN.

VIN > VOUT Operation

The LTC3124 step-up converter will maintain voltage

regulationevenwhentheinputvoltageisabovethedesired

outputvoltage.Notethatoperatinginthismodewillexhibit

lower efficiency and a reduced output current capability.

RefertotheTypicalPerformanceCharacteristicsfordetails.

Burst Mode OPERATION

When the PWM/SYNC pin is held low, the boost converter

operates in Burst Mode, independent of load current. This

mode of operation is typically commanded to improve

efficiency at light loads and reduce standby current at no

load. The output current (IOUT) capability in Burst Mode

operation is significantly less than in PWM mode and

varies with VIN and VOUT, as shown in Figure 5. The logic

input thresholds for this pin are determined relative to VCC

with a low being less than 10% of VCC and a high being

greater than 90% of VCC. The LTC3124 will operate in

fixed frequency PWM mode even if Burst Mode operation

is commanded during soft-start.

In Burst Mode operation, only Phase A of the LTC3124

is operational, while Phase B is disabled. The Phase A

inductor current is initially charged to approximately

700mA by turning on the N-channel MOSFET switch, at

which point the N-channel switch is turned off and the

P-channel synchronous switch is turned on, delivering

currenttotheoutput.Whentheinductorcurrentdischarges

to approximately zero, the cycle repeats. In Burst Mode

operation,energyisdeliveredtotheoutputuntilthenominal

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