LT3840
operaTion
Light Load Operation (Burst Mode Operation,
Pulse-Skipping Mode or Continuous Mode)
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.
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 INTV for continuous mode.
CC
In Burst Mode operation the LT3840 forces a minimum
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.
peak inductor current via an internal clamp on the V pin.
C
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 V volt-
C
age. When the internal voltage clamp on V is engaged
C
and the FB voltage increases slightly, the LT3840 goes
Constant Current Operation
into sleep mode.
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.
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.
The current through the sense resistor, R
, produces
SENSE
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
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.
voltage the ICOMP node is driven high and V is pulled
C
3840fa
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