LTC3858-2
12
38582f
OPERATION
(Refer to the Functional Diagram)
Shutdown and Start-Up (RUN1, RUN2
and SS1, SS2 Pins)
The two channels of the LTC3858-2 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either of
these pins below 1.26V shuts down the main control loop
for that controller. Pulling both pins below 0.7V disables
both controllers and most internal circuits, including the
INTV
CC
LDOs. In this state, the LTC3858-2 draws only 8µA
of quiescent current.
The RUN pin may be externally pulled up or driven directly
by logic. When driving the RUN pin with a low impedance
source, do not exceed the absolute maximum rating of
8V. The RUN pin has an internal 11V voltage clamp that
allows the RUN pin to be connected through a resistor to a
higher voltage (for example, V
IN
), so long as the maximum
current into the RUN pin does not exceed 100µA.
The start-up of each controller’s output voltage, V
OUT
, is
controlled by the voltage on the SS pin for that channel.
When the voltage on the SS pin is less than the 0.8V
internal reference, the LTC3858-2 regulates the V
FB
volt-
age to the SS pin voltage instead of the 0.8V reference.
This allows the SS pin to be used to program a soft-start
by connecting an external capacitor from the SS pin to
SGND. An internal 1µA pull-up current charges this ca-
pacitor creating a voltage ramp on the SS pin. As the SS
voltage rises linearly from 0V to 0.8V (and beyond up to
the absolute maximum rating of 6V), the output voltage
V
OUT
rises smoothly from zero to its final value.
Alternatively, the SS pin can be used to cause the start-
up of V
OUT
to track that of another supply. Typically, this
requires connecting to the SS pin an external resistor
divider from the other supply to ground (see the Applica-
tions Information section).
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Forced Continuous)
(PLLIN/MODE Pin)
The LTC3858-2 can be enabled to enter high efficiency
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
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 amplifier 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 LTC3858-2
draws only 170µA of quiescent current. If both channels
are in sleep mode, the LTC3858-2 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
current comparator, IR, turns off the bottom external
MOSFET just before the inductor current reaches zero,
preventing it from reversing and going negative. Thus,
the controller is in discontinuous operation.
