LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 14

• Manufacturer Part Number: LTC3780EG
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,SSOP,24PIN,PLASTIC
• Manufacturer: Linear Technology

OPERATIO
LTC3780
diode of Synchronous Switch B or the Schottky diode,
which is in parallel in with Synchronous Switch B, is used
to discharge inductor current. As a result, some cycles will
be skipped when the output load current drops below 1%
of the maximum designed load in order to maintain the
output voltage.
When the FCB pin voltage is tied to the INTV
controller enters constant frequency Discontinuous Cur-
rent mode (DCM). For Boost operation, Synchronous
Switch D is held off whenever the I
threshold voltage. In every cycle, Switch C is used to
charge inductor current. After the output voltage is high
enough, the controller will enter continuous current Buck
mode for one cycle to discharge inductor current. In the
following cycle, the controller will resume DCM Boost
operation. For Buck operation, constant frequency Dis-
continuous Current mode sets a minimum negative induc-
tor current level. Synchronous Switch B is turned off
whenever inductor current is lower than this level. At very
light loads, this constant frequency operation is not as
efficient as Burst Mode operation or Skip-Cycle, but does
provide lower noise, constant frequency operation.
FREQUENCY SYNCHRONIZATION AND
FREQUENCY SETUP
The phase-locked loop allows the internal oscillator to be
synchronized to an external source via the PLLIN pin. The
phase detector output at the PLLFLTR pin is also the DC
frequency control input of the oscillator. The frequency
ranges from 200kHz to 400kHz, corresponding to a DC
voltage input from 0V to 2.4V at PLLFLTR. 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 its
minimum frequency.
14
U
TH
pin is below a
CC
pin, the
INTV
Power for all power MOSFET drivers and most internal
circuitry is derived from the INTV
pin is left open, an internal 6V low dropout linear regulator
supplies INTV
6V regulator is turned off and an internal switch is turned
on, connecting EXTV
power to be derived from a high efficiency external source.
POWER GOOD (PGOOD) PIN
The PGOOD pin is connected to an open drain of an internal
MOSFET. The MOSFET turns on and pulls the pin low when
the output is not within ±7.5% of the nominal output level
as determined by the resistive feedback divider. When the
output meets the ±7.5% requirement, the MOSFET is
turned off and the pin is allowed to be pulled up by an
external resistor to a source of up to 7V.
FOLDBACK CURRENT
Foldback current limiting is activated when the output
voltage falls below 70% of its nominal level, reducing power
waste. During start-up, foldback current limiting is disabled.
INPUT UNDERVOLTAGE RESET
The SS capacitor will be reset if the input voltage is allowed
to fall below approximately 4V. The SS capacitor will
attempt to charge through a normal soft-start ramp after
the input voltage rises above 4V.
CC
/EXTV
CC
CC
POWER
power. If EXTV
CC
to INTV
CC
CC
CC
. This allows the INTV
is taken above 5.7V, the
pin. When the EXTV
3780f
CC
CC