LTC3899_15 Datasheet, PDF (15/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3899
Operation (Refer to the Functional Diagrams)
Main Control Loop
The LTC3899 uses a constant frequency, current mode
step-down architecture. The two buck controllers, channels
1 and 2, operate 180° out of phase with each other. The
boost controller, channel 3, operates in phase with chan-
nel 1. During normal operation, the external top MOSFET
for the buck channels (the external bottom MOSFET for
the boost controller) is turned on when the clock for that
channel sets the RS latch, and is turned off when the
main current comparator, I_CMP, resets the RS latch. The
peak inductor current at which I_CMP 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 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 (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 ITH 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 I_R, or the beginning of the next clock cycle.
DRV_CC/EXTV_CC/INTV_CC Power
Power for the top and bottom MOSFET drivers is derived
from the DRV_CC pin. The DRV_CC supply voltage can be
programmed from 5V to 10V through control of the
DRVSET pin. When the EXTV_CC pin is tied to a voltage
below its switchover voltage (4.7V or 7.7V depending
on the DRVSET voltage), the V_BIAS LDO (low dropout
linear regulator) supplies power from V_BIAS to DRV_CC. If
EXTV_CC is taken above its switchover voltage, the V_BIAS
LDO is turned off and an EXTV_CC LDO is turned on. Once
enabled, the EXTV_CC LDO supplies power from EXTV_CC
to DRV_CC. Using the EXTV_CC pin allows the DRV_CC power
to be derived from a high efficiency external source such
as one of the LTC3899 buck regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap capacitor, C_B, which normally recharges during each
cycle through an internal switch whenever SW goes low.
For buck channels 1 and 2, if the 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, resulting in about 99% duty cycle.
The INTV_CC supply powers most of the other internal circuits
in the LTC3899. The INTV_CC LDO regulates to a fixed value
of 5V and its power is derived from the DRV_CC supply.
Shutdown and Start-Up (RUN, TRACK/SS Pins)
The three channels of the LTC3899 can be independently
shut down using the RUN1, RUN2 and RUN3 pins. Pull-
ing a RUN pin 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 DRV_CC and INTV_CC LDOs. In this state, the LTC3899
draws only 3.6μA of quiescent current.
Releasing a RUN pin allows a small 150nA internal current
to pull up the pin to enable that controller. Each RUN pin
may be externally pulled up or driven directly by logic. Each
RUN pin can tolerate up to 65V (absolute maximum), so it
can be conveniently tied to V_BIAS in always-on applications
where one or more controllers are enabled continuously
and never shut down.
The start-up of each controller’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 the 0.8V internal reference for the bucks
and the 1.2V internal reference for the boost, the LTC3899
regulates the V_FB voltage to the TRACK/SS pin voltage
instead of the corresponding reference voltage. This al-
lows the TRACK/SS pin to be used to program a soft-start
by connecting an external capacitor from the TRACK/SS
pin to GND. An internal 10μA pull-up current charges this
capacitor creating a voltage ramp on the TRACK/SS pin. As
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