LTC3786_15 Datasheet, PDF (11/34 Pages) Linear Technology – Low IQ Synchronous Boost Controller

LTC3786
Operation (Refer to the Block Diagram)
Main Control Loop
The LTC3786 uses a constant-frequency, current mode
step-up control architecture. During normal operation,
the external bottom MOSFET is turned on when the clock
sets the RS latch, and is turned off when the main current
comparator, ICMP_ , resets the RS latch. The peak inductor
current at which ICMP trips and resets the latch is con-
trolled by the voltage on the ITH pin, which is the output
of the error amplifier, EA. The error amplifier compares
the output voltage feedback signal at the VFB pin, (which
is generated with an external resistor divider connected
across the output voltage, V_OUT , to ground) to the internal
1.200V reference voltage. In a boost converter, the required
inductor current is determined by the load current, V_IN and
V_OUT . When the load current increases, it causes a slight
decrease in VFB relative to the reference, which causes the
EA to increase the ITH voltage until the average inductor
current in each channel matches the new requirement
based on the new load current.
After the bottom MOSFET is turned off each cycle, the
top MOSFET is turned on until either the inductor current
starts to reverse, as indicated by the current comparator
IR, or the beginning of the next clock cycle.
INTV_CC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV_CC pin. The
VBIAS LDO (low dropout linear regulator) supplies 5.4V
from VBIAS to INTV_CC.
Shutdown and Start-Up (RUN and SS Pins)
The LTC3786 can be shut down using the RUN pin. Pulling
this pin below 1.28V shuts down the main control loop.
Pulling this pin below 0.7V disables the controller and
most internal circuits, including the INTV_CC LDOs. In this
state, the LTC3786 draws only 8µA of quiescent current.
Note: Do not apply load while the chip is in shutdown. The
output MOSFET will be turned off during shutdown and
the output load may cause excessive power dissipation
in the body diode.
The RUN pin may be externally pulled up or driven directly
by logic. When driving the RUN pin with a low imped-
ance 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 resis-
tor to a higher voltage (for example, V_IN), as long as the
maximum current into the RUN pin does not exceed 100µA.
An external resistor divider connected to V_IN can set the
threshold for converter operation. Once running, a 4.5µA
current is sourced from the RUN pin allowing the user to
program hysteresis using the resistor values.
The start-up of the controller’s output voltage, V_OUT , is
controlled by the voltage on the SS pin. When the voltage
on the SS pin is less than the 1.2V internal reference, the
LTC3786 regulates the VFB voltage to the SS pin voltage
instead of the 1.2V reference. This allows the SS pin to
be used to program a soft-start by connecting an external
capacitor from the SS pin to GND. An internal 10µA pull-
up current charges this capacitor creating a voltage ramp
on the SS pin. As the SS voltage rises linearly from 0V to
1.2V, the output voltage rises smoothly to its final value.
Light Load Current Operation—Burst Mode Operation,
Pulse-Skipping or Continuous Conduction
(PLLIN/MODE Pin)
The LTC3786 can be enabled to enter high efficiency Burst
Mode operation, constant-frequency pulse-skipping mode
or forced continuous conduction mode at low load cur-
rents. 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 the controller is enabled for Burst Mode opera-
tion, the minimum peak current in the inductor is set to
approximately 30% 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 the required
current, the error amplifier, EA, will decrease the voltage
on the ITH pin. When the ITH voltage drops below 0.425V,
the internal sleep signal goes high (enabling sleep mode)
and both external MOSFETs are turned off. The ITH pin is
then disconnected from the output of the EA and parked
at 0.450V.
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