LTC3788 Linear Technology, LTC3788 Datasheet - Page 12

• Manufacturer Part Number: LTC3788
• Description: Dual Output Synchronous Boost Controller
• Manufacturer: Linear Technology

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OPERATION
LTC3788
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 amplifi er EA. The error amplifi er
compares the output voltage feedback signal at the VFB
pin, (which is generated with an external resistor divider
connected across the output voltage, V
the internal 1.200V reference voltage. When the load cur-
rent increases, it causes a slight decrease in V
to the reference, which causes the EA to increase the I
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
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
When the EXTV
than 4.8V, the VBIAS LDO (low dropout linear regulator)
supplies 5.4V from VBIAS to INTV
above 4.8V, the VBIAS LDO is turned off and an EXTV
LDO is turned on. Once enabled, the EXTV
12
CC
/EXTV
CC
CC
Power
pin is left open or tied to a voltage less
(Refer to Block Diagram)
CC
. If EXTV
OUT
CC
, to ground) to
LDO supplies
CC
FB
is taken
relative
CC
pin.
TH
CC
5.4V from EXTV
the INTV
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
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
current into the RUN pin does not exceed 100μA.
The start-up of each controller’s output voltage V
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
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
the output voltage V
CC
LDOs. In this state, the LTC3788 draws only 8μA
CC
power to be derived from a high effi ciency
CC
to INTV
OUT
rises smoothly to its fi nal value.
CC
. Using the EXTV
IN
), as long as the maximum
CC
pin allows
FB
voltage
OUT
CC
3788f
is
),