LTC3826 Linear Technology, LTC3826 Datasheet - Page 11

• Manufacturer Part Number: LTC3826
• Description: 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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OPERATION
Main Control Loop
The LTC3826 uses a constant frequency, current mode
step-down architecture with the two controller channels
operating 180 degrees out of phase. During normal opera-
tion, each external top MOSFET is turned on when the clock
for that channel sets the RS latch, and is turned off when
the main current comparator, I
The peak inductor current at which I
the latch is controlled by the voltage on the I
is the output of the error amplifi er EA. The error amplifi er
compares the output voltage feedback signal at the V
pin, (which is generated with an external resistor divider
connected across the output voltage, V
the internal 0.800V 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 top MOSFET is turned off each cycle, the bottom
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 4.7V, an internal 5.25V low dropout linear regulator
supplies INTV
4.7V, the 5.25V regulator is turned off and a 7.5V low
dropout linear regulator is enabled that supplies INTV
power from EXTV
greater than 4.7V), the 7.5V regulator is in dropout and
INTV
is greater than 7.5V (up to an absolute maximum rating
of 10V), INTV
pin allows the INTV
effi ciency external source such as one of the LTC3826
switching regulator outputs.
CC
CC
/EXTV
is approximately equal to EXTV
CC
CC
CC
CC
Power
power from V
is regulated to 7.5V. Using the EXTV
pin is left open or tied to a voltage less
CC
CC
. If EXTV
(Refer to Functional Diagram)
power to be derived from a high
IN
CC
CMP
. If EXTV
is less than 7.5V (but
, resets the RS latch.
CMP
OUT
CC
CC
trips and resets
. When EXTV
, to ground) to
is taken above
TH
pin, which
FB
relative
CC
pin.
TH
CC
CC
CC
FB
Each top MOSFET driver is biased from the fl oating boot-
strap capacitor C
off cycle through an external diode when the top MOSFET
turns off. If the input voltage V
close to V
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
Shutdown and Start-Up (RUN1, RUN2 and TRACK/
SS1, TRACK/SS2 Pins)
The two channels of the LTC3826 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either
of these pins below 0.7V shuts down the main control
loop for that controller. Pulling both pins low disables
both controllers and most internal circuits, including the
INTV
quiescent current.
Releasing either RUN pin allows an internal 0.5μA current
to pull up the pin and enable that controller. Alternatively,
the RUN pin may be externally pulled up or driven directly
by logic. Be careful not to exceed the Absolute Maximum
rating of 7V on this pin.
The start-up of each controller’s output voltage V
controlled by the voltage on the TRACK/SS1 and TRACK/
SS2 pin. When the voltage on the TRACK/SS pin is less
than the 0.8V internal reference, the LTC3826 regulates
the V
0.8V reference. This allows the TRACK/SS pin to be used
to program a soft start by connecting an external capacitor
from the TRACK/SS pin to SGND. An internal 1μA pull-up
current charges this capacitor creating a voltage ramp on
the TRACK/SS pin. As the TRACK/SS voltage rises linearly
from 0V to 0.8V (and beyond), the output voltage V
rises smoothly from zero to its fi nal value.
Alternatively the TRACK/SS pin can be used to cause the
start-up of V
cally, this requires connecting to the TRACK/SS pin an
CC
FB
B
voltage to the TRACK/SS pin voltage instead of the
regulator, and the LTC3826 draws only 4μA of
to recharge.
OUT
OUT
, the loop may enter dropout and attempt
B
to “track” that of another supply. Typi-
, which normally recharges during each
IN
decreases to a voltage
LTC3826
11
OUT
3826fc
OUT
is