LTC3836 LINER [Linear Technology], LTC3836 Datasheet - Page 10

• Manufacturer Part Number: LTC3836
• Description: Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller
• Manufacturer: LINER [Linear Technology]

LTC3836
OPERATION
Main Control Loop
The LTC3836 uses a constant-frequency, current mode
architecture with the two controllers operating 180 degrees
out-of-phase. During normal operation, the top external
power MOSFET is turned on when the clock for its chan-
nel sets the RS latch, and turned off when the current
comparator (I
current at which I
by the voltage on the I
of the error amplifi er (EAMP). The V
output voltage feedback signal from an external resistor
divider. This feedback signal is compared to the internal
0.6V reference voltage by the EAMP . When the load current
increases, it causes a slight decrease in V
0.6V reference, which in turn causes the I
increase until the average inductor current matches the
new load current. While the top N-channel MOSFET is off,
the bottom N-channel MOSFET is turned on until either
the inductor current starts to reverse, as indicated by the
current reversal comparator, I
the next cycle.
Shutdown, Soft-Start and Tracking Start-Up
(RUN/SS and TRACK/SS2 Pins)
The LTC3836 is shut down by pulling the RUN/SS pin
low. In shutdown, all controller functions are disabled and
the chip draws only 6.5μA. The TG and BG outputs are
held low (off) in shutdown. Releasing RUN/SS allows an
internal 0.65μA current source to charge up the RUN/SS
pin. When the RUN/SS pin reaches 0.65V, the LTC3836’s
two controllers are enabled.
The start-up of V
internal soft-start. During soft-start, the error amplifi er
EAMP compares the feedback signal V
soft-start ramp (instead of the 0.6V reference), which rises
linearly from 0V to 0.6V in about 1ms. This allows the
output voltage to rise smoothly from 0V to its fi nal value,
while maintaining control of the inductor current.
The 1ms soft-start time can be increased by con-
necting the optional external soft-start capacitor C
between the RUN/SS and SGND pins. As the RUN/SS
pin continues to rise linearly from approximately 0.65V
to 1.3V (being charged by the internal 0.65μA current
10
CMP
) resets the latch. The peak inductor
CMP
OUT1
TH
(Refer to Functional Diagram)
resets the RS latch is determined
pin, which is driven by the output
is controlled by the LTC3836’s
RCMP
, or the beginning of
FB
FB1
pin receives the
FB
to the internal
relative to the
TH
voltage to
SS
source), the EAMP regulates the V
from 0V to 0.6V.
The start-up of V
TRACK/SS2 pin. When the voltage on the TRACK/SS2
pin is less than the 0.6V internal reference, the LTC3836
regulates the V
instead of the 0.6V reference. This allows the TRACK/SS2
pin to be used to program a soft-start by connecting an
external capacitor from the TRACK/SS2 pin to SGND.
An internal 1μA pull-up current charges this capacitor,
creating a voltage ramp on the TRACK/SS2 pin. As the
TRACK/SS2 voltage rises linearly from 0V to 0.6V (and
beyond), the output voltage V
zero to its fi nal value.
Alternatively, the TRACK/SS2 pin can be used to cause the
start-up of V
cally, this requires connecting to the TRACK/SS2 pin an
external resistor divider from the other supply to ground
(see Applications Information section).
When the RUN/SS pin is pulled low to disable the
LTC3836, or when V
lockout threshold, the TRACK/SS2 pin is pulled low by
an internal MOSFET. When in undervoltage lockout, both
controllers are disabled and the external MOSFETs are
held off.
Light Load Operation (Pulse-Skipping or Continuous
Conduction) (SYNC/FCB Pin)
The LTC3836 can be enabled to enter high effi ciency pulse-
skipping operation or forced continuous conduction mode
at low load currents. To select pulse-skipping operation, tie
the SYNC/FCB pin to a DC voltage above 0.6V (e.g., V
To select forced continuous operation, tie the SYNC/FCB
to a DC voltage below 0.6V (e.g., SGND).
In forced continuous operation, the inductor current is
allowed to reverse at light loads or under large transient
conditions. The peak inductor current is determined by
the voltage on the I
is turned on every cycle (constant-frequency) regardless
of the I
loads is lower than in pulse-skipping operation. However,
continuous mode has the advantages of lower output ripple
and less interference with audio circuitry.
TH
pin voltage. In this mode, the effi ciency at light
OUT2
FB2
OUT2
to “track” that of another supply. Typi-
voltage to the TRACK/SS2 pin voltage
TH
is controlled by the voltage on the
IN
pin. The main N-channel MOSFET
drops below its undervoltage
OUT2
rises smoothly from
FB1
proportionally
3836fa
IN
).