LTC3719EG Linear Technology, LTC3719EG Datasheet - Page 11

• Manufacturer Part Number: LTC3719EG
• Description: IC SW REG SYNC STEP-DOWN 36-SSOP
• Manufacturer: Linear Technology

Specifications of LTC3719EG

• Applications: Controller, AMD Opteron™
• Voltage - Input: 4 ~ 36 V
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 1.55 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

OPERATIO
Main Control Loop
The LTC3719 uses a constant frequency, current mode
step-down architecture with the two output stages oper-
ating 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
inductor current at which I
trolled by the voltage on the I
error amplifier EA. The EAIN pin receives the voltage
feedback signal, which is compared to the internal refer-
ence voltage by the EA. When the load current increases,
it causes a slight decrease in V
reference, which in turn causes the I
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
the beginning of the next cycle.
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
the I
maximum value. As C
voltage is gradually released allowing normal, full-current
operation.
Low Current Operation
The FCB pin selects between two modes of low current
operation. When the FCB pin voltage is below 0.6V, the
controller forces continuous PWM current mode opera-
OUT
SS
reaches 1.5V, the main control loop is enabled with
, the loop may enter dropout and attempt to turn on
TH
voltage clamped at approximately 30% of its
B
U
, which normally is recharged during
SS
1
(Refer to Functional Diagram)
, resets the RS latch. The peak
IN
continues to charge, the I
decreases to a voltage close to
1
B
TH
resets the RS latch is con-
to recharge.
pin, which is the output of
EAIN
TH
relative to the 0.6V
voltage to increase
SS
. When
TH
2
, or
pin
tion. 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
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
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
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 amplifier
gain block.
Constant Frequency Operation
When the FCB pin is tied to INTV
is disabled and a forced minimum peak output current
requirement 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.
Continuous Current (PWM) Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least efficient 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 be
forced back into the main power supply potentially boost-
ing the input supply to dangerous voltage levels—
BEWARE!
TH
pin below a voltage threshold that will
INTVCC
– 2V but greater than 0.6V, the
CC
, Burst Mode operation
TH
pin. This hysteresis
LTC3719
sn3719 3719fs
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