LTC3707 Linear Technology, LTC3707 Datasheet - Page 10

• Manufacturer Part Number: LTC3707
• Description: High Efficiency / 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

OPERATIO
LTC3707
Main Control Loop
The LTC3707 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 top MOSFET is turned on when the clock for that
channel sets the RS latch, and turned off when the main
current comparator, I
inductor current at which I
trolled by the voltage on the I
each error amplifier EA. The V
voltage feedback signal, which is compared to the internal
reference voltage by the EA. When the load current in-
creases, it causes a slight decrease in V
the 0.8V reference, which in turn causes the I
increase until the average inductor current matches the
new load current. After the top MOSFET has turned off, the
bottom MOSFET is turned on until either the inductor
current starts to reverse, as indicated by current compara-
tor I
The top MOSFET drivers are biased from floating boot-
strap capacitor C
each off cycle through an external diode when the top
MOSFET turns off. As V
V
the top MOSFET continuously. The dropout detector de-
tects this and forces the top MOSFET off for about 500ns
every tenth cycle to allow C
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 1.2 A
current source to charge soft-start capacitor C
C
I
value. As C
gradually released allowing normal, full-current opera-
tion. When both RUN/SS1 and RUN/SS2 are low, all
LTC3707 controller functions are shut down, and the
STBYMD pin determines if the standby 5V and 3.3V
regulators are kept alive.
10
TH
OUT
SS
voltage clamped at approximately 30% of its maximum
reaches 1.5V, the main control loop is enabled with the
2
, the loop may enter dropout and attempt to turn on
, or the beginning of the next cycle.
SS
continues to charge, the I
B
U
, which normally is recharged during
(Refer to Functional Diagram)
1
, resets the RS latch. The peak
IN
decreases to a voltage close to
1
B
TH
resets the RS latch is con-
to recharge.
pin, which is the output of
OSENSE
OSENSE
pin receives the
TH
pin voltage is
TH
relative to
voltage to
SS
. When
Low Current Operation
The FCB pin is a multifunction pin providing two func-
tions: 1) an analog input to provide regulation for a
secondary winding by temporarily forcing continuous
PWM operation on both controllers and 2) a logic input
to select between two modes of low current operation.
When the FCB pin voltage is below 0.800V, the controller
forces continuous PWM current mode operation. In this
mode, the top and bottom MOSFETs are alternately
turned on to maintain the output voltage independent of
direction of inductor current. When the FCB pin is below
V
ters Burst Mode operation. Burst Mode operation sets a
minimum output current level before inhibiting the top
switch and turns off the synchronous MOSFET(s) when
the inductor current goes negative. This combination of
requirements will, at low currents, force the I
a voltage threshold that will temporarily inhibit turn-on of
both output MOSFETs until the output voltage drops.
There is 60mV of hysteresis in the burst comparator B
tied to the I
signals to the MOSFETs that turn them on for several
cycles, followed by a variable “sleep” interval depending
upon the load current. The resultant output voltage ripple
is held to a very small value by having the hysteretic
comparator after the error amplifier gain block.
Constant Frequency Operation
When the FCB pin is tied to INTV
is disabled and the forced minimum output current re-
quirement is removed. This provides constant frequency,
discontinuous (preventing reverse inductor current) cur-
rent operation over the widest possible output current
range. This constant frequency operation is not as efficient
as Burst Mode operation, but does provide a lower noise,
constant frequency operating mode down to approxi-
mately 1% of designed maximum output current. Voltage
should not be applied to the FCB pin prior to the application
of voltage to the V
INTVCC
– 2V but greater than 0.80V, the controller en-
TH
pin. This hysteresis produces output
IN
pin.
CC
, Burst Mode operation
TH
pin below
3707f