LTC3859_15 Datasheet, PDF (15/42 Pages) Linear Technology – Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3859
OPERATION (Refer to Functional Diagram)
Main Control Loop
The LTC3859 uses a constant frequency, current mode
step-down architecture. The two buck controllers, chan-
nels 1 and 2, operate 180 degrees out of phase with each
other. The boost controller, channel 3, operates in phase
with channel 1. During normal operation, the external
top MOSFET for the buck channels (the external bottom
MOSFET for the boost channel) 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 I_TH pin,
which is the output of the error amplifier EA. The error
amplifier compares the output voltage feedback signal at
the V_FB pin, (which is generated with an external resistor
divider connected across the output voltage, V_OUT, to
ground) to the internal 0.800V reference voltage for the
bucks (1.2V reference voltage for the boost). When the
load current 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 top MOSFET for the bucks (the bottom MOSFET
for the boost) is turned off each cycle, the bottom MOSFET
is turned on (the top MOSFET for the boost) 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 4.7V, the V_BIAS LDO (low dropout linear regulator)
supplies 5.4V from V_BIAS to INTV_CC. If EXTV_CC is taken
above 4.7V, the V_BIAS 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 LTC3859 switching
regulator outputs.
Each top MOSFET driver is biased from the floating
bootstrap capacitor C_B, which normally recharges during
each cycle through an external diode when the switch
voltage goes low.
For buck channels 1 and 2, if the buck’s input voltage
decreases to a voltage close to its output, the loop may
enter dropout and attempt to turn on the top MOSFET
continuously. The dropout detector detects this and forces
the top MOSFET off for about one twelfth of the clock
period every tenth cycle to allow C_B to recharge.
Shutdown and Start-Up (RUN1, RUN2, RUN3 and
TRACK/SS1, TRACK/SS2, SS3 Pins)
The three channels of the LTC3859 can be independently
shut down using the RUN1, RUN2 and RUN3 pins. Pulling
any of these pins below 1.19V shuts down the main control
loop for that channel. Pulling all three pins below 0.7V
disables all controllers and most internal circuits, including
the INTV_CC LDOs. In this state, the LTC3859 draws only
14μA of quiescent current.
Releasing a RUN pin allows a small internal current to pull
up the pin to enable that controller. The RUN1 pin has a
6μA pull-up current while the RUN2 and RUN3 pins have
a smaller 0.5μA. The 6μA current on RUN1 is designed
to be large enough so that the RUN1 pin can be safely
floated (to always enable the controller) without worry
of condensation or other small board leakage pulling the
pin down. This is ideal for always-on applications where
one or more controllers are enabled continuously and
never shut down.
Each RUN pin may also be externally pulled up or driven
directly by logic. When driving a RUN pin with a low
impedance source, do not exceed the absolute maximum
rating of 8V. Each 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_BIAS), so
long as the maximum current in the RUN pin does not
exceed 100μA.
The start-up of each channel’s output voltage V_OUT is
controlled by the voltage on the TRACK/SS pin (TRACK/SS1
for channel 1, TRACK/SS2 for channel 2, SS3 for channel 3).
When the voltage on the TRACK/SS pin is less than
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