LTC3736 LINER [Linear Technology], LTC3736 Datasheet - Page 10

• Manufacturer Part Number: LTC3736
• Description: Dual 2-Phase, No RSENSE Synchronous Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC3736
Main Control Loop
The LTC3736 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 is shut down by pulling the RUN/SS pin low.
In shutdown, all controller functions are disabled and the
chip draws only 9 A. The TG outputs are held high (off)
and the BG outputs low (off) in shutdown. Releasing
RUN/SS allows an internal 0.7 A current source to charge
up the RUN/SS pin. When the RUN/SS pin reaches 0.65V,
the LTC3736’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’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 0.7 A 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 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 (Burst Mode or Continuous
Conduction) (SYNC/FCB Pin)
The LTC3736 can be enabled to enter high efficiency Burst
Mode operation or forced continuous conduction mode at
low load currents. To select Burst Mode operation, tie the
SYNC/FCB pin to a DC voltage above 0.6V (e.g., V
select forced continuous operation, tie the SYNC/FCB to a
DC voltage below 0.6V (e.g., SGND). This 0.6V threshold
between Burst Mode operation and forced continuous mode
can be used in secondary winding regulation as described
in the Auxiliary Winding Control Using SYNC/FCB Pin dis-
cussion in the Applications Information section.
When a controller is in Burst Mode operation, the peak
current in the inductor is set to approximate one-fourth of
the maximum sense voltage even though the voltage on
the I
current is lower than the load current, the EAMP will
decrease the voltage on the I
drops below 0.85V, the internal SLEEP signal goes high
and both external MOSFETs are turned off.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3736 draws.
The load current is supplied by the output capacitor. As
the output voltage decreases, the EAMP increases the I
voltage. When the I
signal goes low and the controller resumes normal
operation by turning on the external P-channel MOSFET
on the next cycle of the internal oscillator.
TH
FB2
pin indicates a lower value. If the average inductor
voltage to the TRACK pin instead of the 0.6V
OUT2
FB1
OUT1
OUT2
that is connected to V
proportionally linearly from 0V to 0.6V.
TH
. For one-to-one tracking during start-
voltage reaches 0.925V, the SLEEP
is controlled by the voltage on the
TH
pin. When the I
FB2
.
OUT1
TH
voltage
is con-
OUT2
IN
). To
3736f
TH
to