LTC3788-1 Datasheet, PDF (11/28 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788-1
OPERATION (Refer to Block Diagram)
Main Control Loop
The LTC3788-1 uses a constant-frequency, current mode
step-up architecture with the two controller channels oper-
ating 180 degrees out-of-phase. During normal operation,
each external bottom MOSFET is turned on when the clock
for that channel 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 controlled 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. When the load cur-
rent increases, it causes a slight decrease in V_FB relative
to the reference, which causes the EA to increase the I_TH
voltage until the average inductor current matches 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/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV_CC pin.
When the EXTV_CC pin is left open or tied to a voltage less
than 4.8V, the VBIAS LDO (low dropout linear regulator)
supplies 5.4V from VBIAS to INTV_CC. If EXTV_CC is taken
above 4.8V, the VBIAS LDO is turned off and an EXTV_CC
LDO is turned on. Once enabled, the EXTV_CC LDO supplies
5.4V from EXTV_CC to INTV_CC. Using the EXTV_CC pin allows
the INTV_CC power to be derived from a high efficiency
external source such as one of the LTC3788-1 switching
regulator outputs.
Shutdown and Start-Up
(RUN1, RUN2 and SS1, SS2 Pins)
The two channels of the LTC3788-1 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either of
these pins below 1.28V 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 LDO’s. In this state, the LTC3788-1 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), as 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 1.2V
internal reference, the LTC3788-1 regulates the V_FB volt-
age 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
SGND. 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 (and beyond
up to INTV_CC), the output voltage V_OUT rises smoothly
to its final value.
Light Load Current Operation—Burst Mode Operation,
Pulse-Skipping or Continuous Conduction
(PLLIN/MODE Pin)
The LTC3788-1 can be enabled to enter high efficiency
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 a ground (e.g., SGND). To se-
lect 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 – 0.5V.
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 ITH 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 ITH
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.
37881f
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