LTC3736-2 Linear Technology, LTC3736-2 Datasheet - Page 10

• Manufacturer Part Number: LTC3736-2
• Description: Synchronous Controller
• Manufacturer: Linear Technology

OPERATIO
LTC3736-2
Main Control Loop
The LTC3736-2 uses a constant frequency, current mode
architecture with the two controllers operating 180 de-
grees out of phase. During normal operation, the top
external P-channel power MOSFET is turned on when the
clock for that channel sets the RS latch, and turned off
when the current comparator (I
peak inductor current at which I
determined by the voltage on the I
by the output of the error amplifier (EAMP). The V
receives the output voltage feedback signal from an exter-
nal 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
relative to the 0.6V reference, which in turn causes the I
voltage to increase until the average inductor current
matches the new load current. While the top P-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
beginning of the next cycle.
Shutdown, Soft-Start and Tracking Start-Up
(RUN/SS and TRACK Pins)
The LTC3736-2 is shut down by pulling the RUN/SS pin
low. In shutdown, all controller functions are disabled and
the chip draws only 9µA. The TG outputs are held high (off)
and the BG outputs low (off) in shutdown. Releasing
RUN/SS allows an internal 0.7µA current source to charge
up the RUN/SS pin. When the RUN/SS pin reaches 0.65V,
the LTC3736-2’s two controllers are enabled.
The start-up of V
internal soft-start. During soft-start, the error amplifier
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 final value,
while maintaining control of the inductor current.
The 1ms soft-start time can be increased by connecting
the optional external soft-start capacitor C
RUN/SS and SGND pins. As the RUN/SS pin continues to
10
U
OUT1
(Refer to Functional Diagram)
is controlled by the LTC3736-2’s
CMP
CMP
TH
) resets the latch. The
resets the RS latch is
pin, which is driven
FB1
SS
to the internal
between the
RCMP
, or the
FB
pin
TH
FB
rise linearly from approximately 0.65V to 1.3V (being
charged by the internal 0.7µA current source), the EAMP
regulates the V
The start-up of V
TRACK pin. When the voltage on the TRACK pin is less
than the 0.6V internal reference, the LTC3736-2 regulates
the V
reference. Typically, a resistor divider on V
nected to the TRACK pin to allow the start-up of V
“track” that of V
up, the resistor divider would have the same values as the
divider on V
Light Load Operation (Pulse-Skipping or Continuous
Conduction) (SYNC/FCB Pin)
The LTC3736-2 can be enabled to enter high efficiency
pulse-skipping operation or forced continuous conduc-
tion 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
SYNC/FCB to a DC voltage below 0.6V (e.g., SGND). This
0.6V threshold between pulse-skipping operation and
forced continuous mode can be used in secondary wind-
ing regulation as described in the Auxiliary Winding Con-
trol Using SYNC/FCB Pin discussion in the Applications
Information section.
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
every cycle (constant frequency) regardless of the I
voltage. In this mode, the efficiency at light loads is lower
than in pulse-skipping operation. However, continuous
mode has the advantages of lower output ripple and less
interference with audio circuitry.
When the SYNC/FCB pin is tied to a DC voltage above 0.6V
or when it is clocked by an external clock source to use the
phase-locked loop (see Frequency Selection and Phase-
Locked Loop), the LTC3736-2 operates in PWM pulse-
skipping mode at light loads. In this mode, the current
comparator I
FB2
IN
). To select forced continuous operation, tie the
voltage to the TRACK pin instead of the 0.6V
OUT2
CMP
FB1
TH
OUT1
may remain tripped for several cycles and
OUT2
that is connected to V
pin. The P-channel MOSFET is turned on
proportionally linearly from 0V to 0.6V.
. For one-to-one tracking during start-
is controlled by the voltage on the
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FB2
.
OUT1
OUT2
is con-
TH
37362fa
pin
to