LTC3866 Datasheet, PDF (12/36 Pages) Linear Technology – Current Mode Synchronous Controller for Sub Milliohm DCR Sensing

LTC3866
Operation
Shutdown and Start-Up (RUN and TK/SS Pins)
The LTC3866 can be shut down using the RUN pin. Pulling
the RUN pin below 1.14V shuts down the main control loop
for the controller and most internal circuits, including the
INTV_CC regulator. Releasing the RUN pin allows an internal
1.0µA current to pull up the pin and enable the controller.
Alternatively, the RUN pin may be externally pulled up
or driven directly by logic. Be careful not to exceed the
absolute maximum rating of 6V on this pin. The start-up
of the controller’s output voltage, V_OUT , is controlled by
the voltage on the TK/SS pin, if the internal soft-start
has expired. When the voltage on the TK/SS pin is less
than the 0.6V internal reference, the LTC3866 regulates
the V_FB voltage to the TK/SS pin voltage instead of the
0.6V reference. This allows the TK/SS pin to be used to
program a soft-start by connecting an external capacitor
from the TK/SS pin to SGND. An internal 1.25µA pull-up
current charges this capacitor, creating a voltage ramp on
the TK/SS pin. As the TK/SS voltage rises linearly from
0V to 0.6V (and beyond), the output voltage, V_OUT , rises
smoothly from zero to its final value. Alternatively, the
TK/SS pin can be used to cause the start-up of V_OUT to track
that of another supply. Typically, this requires connect-
ing to the TK/SS pin an external resistor divider from the
other supply to ground (see the Applications Information
section). When the RUN pin is pulled low to disable the
controller, or when INTV_CC drops below its undervoltage
lockout threshold of 3.75V, the TK/SS pin is pulled low by
an internal MOSFET. When in undervoltage lockout, the
controller is disabled and the external MOSFETs are held off.
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Continuous Conduction)
The LTC3866 can be enabled to enter high efficiency Burst
Mode operation, constant-frequency pulse-skipping mode
or forced continuous conduction mode. To select forced
continuous operation, tie the MODE/PLLIN pin to SGND.
To select pulse-skipping mode of operation, tie the MODE/
PLLIN pin to INTV_CC. To select Burst Mode operation, float
the MODE/PLLIN pin. When the controller is enabled for
Burst Mode operation, the peak current in the inductor
is set to approximately one-third of the maximum sense
voltage even though the voltage on the ITH pin indicates a
lower value. If the average inductor current is higher than
12
the load current, the error amplifier, EA, will decrease the
voltage on the ITH pin. When the ITH voltage drops below
0.5V, the internal sleep signal goes high (enabling “sleep”
mode) and both external MOSFETs are turned off.
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
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 the controller
is enabled for Burst Mode operation, the inductor current
is not allowed to reverse. The reverse current comparator
(I_REV) turns off the bottom external MOSFET just before
the inductor current reaches zero, preventing it from re-
versing and going negative. Thus, the controller operates
in discontinuous operation.
In forced continuous operation, the inductor current is
allowed to reverse at light loads or under large transient
conditions. The peak inductor current is determined by
the voltage on the ITH pin, just as in normal operation.
In this mode, the efficiency at light loads is lower than in
Burst Mode operation. However, continuous mode has the
advantages of lower output ripple and less interference
with audio circuitry.
When the MODE/PLLIN pin is connected to INTV_CC, the
LTC3866 operates in PWM pulse skipping mode at light
loads. At very light loads, the current comparator, I_CMP ,
may remain tripped for several cycles and force the external
top MOSFET to stay off for the same number of cycles (i.e.,
skipping pulses). The inductor current is not allowed to
reverse (discontinuous operation). This mode, like forced
continuous operation, exhibits low output ripple as well as
low audio noise and reduced RF interference as compared
to Burst Mode operation. It provides higher low current
efficiency than forced continuous mode, but not nearly as
high as Burst Mode operation.
Frequency Selection and Phase-Locked Loop
(FREQ and MODE/PLLIN Pins)
The selection of switching frequency is a trade-off between
efficiency and component size. Low frequency opera-
tion increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage.
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