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LT3840

3840fa

For more information www.linear.com/LT3840

Light Load Operation (Burst Mode Operation,

Pulse-Skipping Mode or Continuous Mode)

The LT3840 is capable of operating in Burst Mode, pulse-

skipping mode, or continuous mode. Connect the MODE

pin to GND for Burst Mode operation, to the FB pin for

pulse-skipping mode, or to INTVCC for continuous mode.

In Burst Mode operation the LT3840 forces a minimum

peak inductor current via an internal clamp on the VC pin.

If the average inductor current is greater than the load

current the output voltage will begin to increase and the

error amplifier, EA, will attempt to decrease the VC volt-

age. When the internal voltage clamp on VC is engaged

and the FB voltage increases slightly, the LT3840 goes

into sleep mode.

In sleep mode, both external MOSFETs are turned off

and much of the internal circuitry is turned off, reducing

the quiescent current. The load current is supplied by

the output capacitor. As the output voltage decreases,

the LT3840 comes out of sleep mode and the controller

resumes normal operation by turning on the TG MOSFET

on the next cycle of the internal oscillator. The output

voltage increases and the controller goes back to sleep.

This cycle repeats until the average load current is greater

than the minimum forced peak inductor current.

When Burst Mode operation is selected, the inductor

current is not allowed to go negative. A reverse current

comparator turns off the BG MOSFET just before the

inductor current reaches zero, preventing it from revers-

ing and going negative. Thus, the controller operates in

discontinuous operation.

In pulse-skipping mode, during light loads, the supply

operates in discontinuous mode where the inductor cur-

rent is not allowed to reverse direction. Output voltage

regulation is maintained by skipping TG on pulses. At

light loads pulse-skipping mode is more efficient than

forced continuous mode, but not as efficient as Burst

Mode operation.

In continuous operation the inductor current is allowed

to reverse direction at light loads or under large transient

conditions. The reverse current comparator protects the

BG MOSFET by turning it off if the reverse current exceeds

the maximum reverse current sense threshold voltage.

Constant Current Operation

For applications requiring a regulated current source the

LT3840 has a control loop to accurately regulate the aver-

age output current. A current monitor function provides

output current information for telemetry and diagnostics.

The current through the sense resistor, RSENSE, produces

a voltage applied to the SENSE pins. The differential sense

voltage is amplified by 20x, buffered and output to the

IMON pin. The capacitor on the IMON pin filters the ripple

component to average the signal.

The 20x amplified differential sense voltage is also applied

to an internal GM amplifier and compared against either

1V or ICTRL voltage, whichever is smaller. A voltage

applied to ICTRL reduces the maximum average current

sensethreshold.Whenthe20×amplifieddifferentialsense

voltage exceeds the 1V internal reference or the ICTRL

voltage the ICOMP node is driven high and VC is pulled

operaTion

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