Datasheet

LTC3868-1
12
38681fd
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 LTC3868-1 have sepa-
rate, 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 resis-
tor also shortens the soft-start period.
Foldback Current
On the other hand, when the output voltage falls to less
than 70% 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 LTC3868-1 can be enabled to enter high effi ciency
Burst Mode operation, constant frequency pulse-skipping
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 opera-
tion, 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 ap-
proximately 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 LTC3868-1
draws only 170µA of quiescent current. If both channels
are in sleep mode, the LTC3868-1 draws only 300µA of qui-
escent 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.
When a controller is enabled for Burst Mode operation,
the inductor current is not allowed to reverse. The reverse
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
38681 F01
1µA
Figure 1. Latchoff Timing Diagram