LTC3859AL_15 Datasheet, PDF (15/44 Pages) Linear Technology – Triple Output, Buck/Buck/Boost Synchronous Controller with 28A Burst Mode IQ

LTC3859AL
Operation (Refer to Functional Diagram)
Main Control Loop
The LTC3859AL 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 LTC3859AL switching
regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap 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 con-
tinuously. 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 LTC3859AL can be independently
shut down using the RUN1, RUN2 and RUN3 pins. Pulling
RUN1 below 1.17V and RUN2/RUN3 below 1.20V 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 LTC3859AL draws only 10µA 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
7µA pull-up current while the RUN2 and RUN3 pins have
a smaller 160nA. The 7µA 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 imped-
ance 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 con-
trolled 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 the
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3859alf
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