Datasheet

LTC3868
12
3868fd
OPERATION
(Refer to the Functional Diagram)
The delay time from when a short-circuit occurs until the
controller latches off can be calculated using the follow-
ing equation:
t
LATCH
~ C
SS
(V
SS
– 1.5V)/9µA
where V
SS
is the initial voltage (must be greater than 2V)
on the SS pin at the time the short-circuit occurs. Normally
the SS pin voltage will have been pulled up to the INTV
CC
voltage (5.1V) by the internal 1µA pull-up current.
Note that the two controllers on the LTC3868 have separate,
independent short-circuit latchoff circuits. Latchoff can be
overridden/defeated by connecting a resistor 150k or less
from the SS pin to INTV
CC
. This resistor provides enough
pull-up current to overcome the 9µA pull-down current
present during a short-circuit. Note that this resistor also
shortens the soft-start period.
Foldback Current
On the other hand, when the output voltage falls to less
than 72% of its nominal level, foldback current limiting
is also activated, progressively lowering the peak current
limit in proportion to the severity of the overcurrent or
short-circuit condition. Even if a short-circuit is present
and the short-circuit latchoff is not yet enabled (when
SS voltage has not yet reached 2V), a safe, low output
current is provided due to internal current foldback and
actual power wasted is low due to the effi cient nature of
the current mode switching regulator. Foldback current
limiting is disabled during the soft-start interval (as long
as the V
FB
voltage is keeping up with the SS voltage).
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Forced Continuous Mode)
(PLLIN/MODE Pin)
The LTC3868 can be enabled to enter high effi ciency
Burst Mode operation, constant frequency pulse-skip-
ping mode, or forced continuous conduction mode at
low load currents. To select Burst Mode operation, tie the
PLLIN/ MODE pin to ground. To select forced continuous
operation, tie the PLLIN/MODE pin to INTV
CC
. To select
pulse-skipping mode, tie the PLLIN/MODE pin to a DC
voltage greater than 1.2V and less than INTV
CC
– 1.3V.
When a 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 I
TH
pin indicates a lower value. If the
average inductor current is higher than the load current,
the error amplifi er EA will decrease the voltage on the I
TH
pin. When the I
TH
voltage drops below 0.425V, the internal
sleep signal goes high (enabling sleep mode) and both
external MOSFETs are turned off.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current. If one channel is shut down
and the other channel is in sleep mode, the LTC3868 draws
only 170µA of quiescent current. If both channels are in
sleep mode, the LTC3868 draws only 300µA of quiescent
current. In sleep mode, the load current is supplied by
the output capacitor. As the output voltage decreases, the
EA’s output begins to rise. When the output voltage drops
enough, the I
TH
pin is reconnected to the output of the
EA, the sleep signal goes low, and the controller resumes
normal operation by turning on the top external MOSFET
on the next cycle of the internal oscillator.
INTV
CC
2V
0.8V
1.5V
0V
1µA
–9µA
SS VOLTAGE
LATCHOFF
COMMAND
SS PIN
CURRENT
OUTPUT
VOLTAGE
LATCHOFF
ENABLE
SOFT-START INTERVAL
ARMING
t
LATCH
3868 F01
1µA
Figure 1. Latchoff Timing Diagram