LTC1628CG-SYNC#TR Linear Technology, LTC1628CG-SYNC#TR Datasheet - Page 10

• Manufacturer Part Number: LTC1628CG-SYNC#TR
• Description: IC REG SW 2PH SYNC STPDWN 28SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1628CG-SYNC#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: Adj to 0.8V
• Current - Output: 3A
• Frequency - Switching: 140kHz ~ 310kHz
• Voltage - Input: 3.5 ~ 30 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

OPERATIO
LTC1628-SYNC
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 400ns
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 1.2µA
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
LTC1628-SYNC controller functions are shut down,
including the 5V and 3.3V regulators.
Low Current Operation
The FCB pin is a multifunction pin providing two func-
tions: 1) to provide regulation for a secondary winding by
temporarily forcing continuous PWM operation on
both controllers; and 2) select between two modes of low
current operation. When the FCB pin voltage is below
0.8V, 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
0.8V, the controller enters Burst Mode operation. Burst
Mode operation sets a minimum output current level
before inhibiting the top switch and turns off the synchro-
nous MOSFET(s) when the inductor current goes nega-
tive. This combination of requirements will, at low cur-
rents, force the I
temporarily inhibit turn-on of both output MOSFETs until
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
TH
B
U
, which normally is recharged during
pin below a voltage threshold that will
(Refer to Functional Diagram)
IN
decreases to a voltage close to
B
INTVCC
to recharge.
– 2V but greater than
TH
pin voltage is
SS
. When
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.
Frequency Synchronization
The phase-locked loop allows the internal oscillator to be
synchronized to an external source via the PLLIN pin. The
output of the phase detector at the PLLFLTR pin is also the
DC frequency control input of the oscillator that operates
over a 140kHz to 310kHz range corresponding to a DC
voltage input from 0V to 2.4V. When locked, the PLL aligns
the turn on of the top MOSFET to the rising edge of the
synchronizing signal. When PLLIN is left open, the PLLFLTR
pin goes low, forcing the oscillator to minimum frequency.
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!
INTV
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
When the EXTV
dropout linear regulator supplies INTV
is taken above 4.7V, the 5V regulator is turned off and an
internal switch is turned on connecting EXTV
This allows the INTV
efficiency external source such as the output of the regu-
lator itself or a secondary winding, as described in the
Applications Information section.
CC
/EXTV
CC
Power
CC
pin is left open, an internal 5V low
CC
power to be derived from a high
TH
pin. This hysteresis
CC
power. If EXTV
CC
to INTV
CC
1628syncfa
pin.
CC
CC
.