ltc3827ig-1 Linear Technology Corporation, ltc3827ig-1 Datasheet - Page 9

• Manufacturer Part Number: ltc3827ig-1
• Description: Low Iq, Dual, 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

OPERATIO
Main Control Loop
The LTC3827-1 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
latch. The peak inductor current at which I
resets the latch is controlled by the voltage on the I
which is the output of the error amplifier EA. The error
amplifier compares the output voltage feedback signal at
the V
divider connected across the output voltage, V
ground) to the internal 0.800V reference voltage. When
the load current increases, it causes a slight decrease in
V
increase the I
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
allows the INTV
efficiency external source such as one of the LTC3827-1
switching regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap capacitor C
off cycle through an external diode when the top MOSFET
turns off. If the input voltage V
FB
relative to the reference, which causes the EA to
CC
CC
FB
/EXTV
pin, (which is generated with an external resistor
is approximately equal to EXTV
CC
TH
CC
CC
is regulated to 7.5V. Using the EXTV
CC
voltage until the average inductor current
Power
power from V
B
CC
U
, which normally recharges during each
pin is left open or tied to a voltage less
CC
power to be derived from a high
. If EXTV
(Refer to Functional Diagram)
IN
CC
IN
. If EXTV
is less than 7.5V (but
decreases to a voltage
CMP
CC
CC
. When EXTV
, resets the RS
is taken above
CMP
trips and
OUT
CC
TH
CC
pin,
, to
pin.
pin
CC
CC
close to V
turn on the top MOSFET continuously. The dropout detec-
tor detects this and forces the top MOSFET off for about
one twelfth of the clock period every tenth cycle to allow C
to recharge.
Shutdown and Start-Up (RUN1, RUN2 and TRACK/
SS1, TRACK/SS2 Pins)
The two channels of the LTC3827-1 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
trolled 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 LTC3827-1 regulates the V
voltage to the TRACK/SS pin voltage instead of the 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 final value.
Alternatively the TRACK/SS pin can be used to cause the
startup of V
this requires connecting to the TRACK/SS pin an external
resistor divider from the other supply to ground (see
Applications Information section).
When the corresponding RUN pin is pulled low to disable
a controller, or when V
lockout threshold of 3.5V, the TRACK/SS pin is pulled low
by an internal MOSFET. When in undervoltage lockout,
both controllers are disabled and the external MOSFETs
are held off.
CC
regulator, and the LTC3827-1 draws only 8μA of
OUT
OUT
, the loop may enter dropout and attempt to
to “track” that of another supply. Typically,
IN
drops below its undervoltage
LTC3827-1
OUT
is con-
38271fd
9
OUT
FB
B