Datasheet

LTC3707
10
3707fb
OPERATION
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
operation, 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
1
, resets the RS latch.
The peak inductor current at which I
1
resets the RS
latch is controlled by the voltage on the I
TH
pin, which
is the output of each error amplifi er EA. The V
OSENSE
pin
receives the voltage feedback signal, which is compared
to the internal reference voltage by the EA. When the load
current increases, it causes a slight decrease in V
OSENSE
relative to the 0.8V reference, which in turn causes the
I
TH
voltage to 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
comparator I
2
, or the beginning of the next cycle.
The top MOSFET drivers are biased from fl oating bootstrap
capacitor C
B
, which normally is recharged during each off
cycle through an external diode when the top MOSFET
turns off. As V
IN
decreases to a voltage close to V
OUT
,
the loop may enter dropout and attempt to turn on the
top MOSFET continuously. The dropout detector detects
this and forces the top MOSFET off for about 500ns every
tenth cycle to allow C
B
to recharge.
The main control loop is shut down by pulling the RUN/
SS pin low. Releasing RUN/SS allows an internal 1.2µA
current source to charge soft-start capacitor C
SS
. When
C
SS
reaches 1.5V, the main control loop is enabled with the
I
TH
voltage clamped at approximately 30% of its maximum
value. As C
SS
continues to charge, the I
TH
pin voltage is
gradually released allowing normal, full-current operation.
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.
Low Current Operation
The FCB pin is a multifunction pin providing two
functions: 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
INTVCC
– 2V but greater than 0.80V, the controller
enters 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
TH
pin below
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
TH
pin. This hysteresis produces output 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 amplifi er gain block.
Constant Frequency Operation
When the FCB pin is tied to INTV
CC
, Burst Mode operation
is disabled and the forced minimum output current
requirement is removed. This provides constant frequency,
discontinuous (preventing reverse inductor current)
current operation over the widest possible output current
range. This constant frequency operation is not as effi cient
as Burst Mode operation, but does provide a lower noise,
constant frequency operating mode down to approximately
1% of designed maximum output current. Voltage should
not be applied to the FCB pin prior to the application of
voltage to the V
IN
pin.
Continuous Current (PWM) Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least effi cient operating mode,
but may be desirable in certain applications. The output
can source or sink current in this mode. When sinking
current while in forced continuous operation, current will
(Refer to Functional Diagram)