LTC3788 Datasheet, PDF (12/32 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788
OPERATION (Refer to Block Diagram)
Main Control Loop
The LTC3788 uses a constant-frequency, current mode
step-up architecture with the two controller channels oper-
ating 180 or 240 degrees out-of-phase (depending on the
PHASMD pin connection). 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 switching
regulator outputs.
Shutdown and Start-Up
(RUN1, RUN2 and SS1, SS2 Pins)
The two channels of the LTC3788 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 LDOs. In this state, the LTC3788 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 regulates the V_FB 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 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.
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