LTC1709-9_15 Datasheet, PDF (10/28 Pages) Linear Technology – 2-Phase, 5-Bit VID, Current Mode, High Efficiency, Synchronous Step-Down Switching Regulators

LTC1709-8/LTC1709-9
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
The LTC1709 uses a constant frequency, current mode
step-down architecture with inherent current sharing.
During normal operation, the top MOSFET is turned on
each cycle when the oscillator sets the RS latch, and
turned off when the main current comparator, I₁, resets
the RS latch. The peak inductor current at which I₁ resets
the RS latch is controlled by the voltage on the I_TH pin,
which is the output of the error amplifier EA. The differen-
tial amplifier, A1, produces a signal equal to the differen-
tial voltage sensed across the output capacitor but
re-references it to the internal signal ground (SGND)
reference. The EAIN pin receives a portion of this voltage
feedback signal at the DIFFOUT as determined by VID
logic input pins (VID0 to VID4) and is compared to the
internal reference voltage by the EA. When the load
current increases, it causes a slight decrease in the EAIN
pin voltage 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 for the rest of the period.
The top MOSFET drivers are biased from floating boot-
strap capacitor C_B, which normally is recharged during
each off cycle through an external Schottky diode. When
V_IN decreases to a voltage close to V_OUT, however, the loop
may enter dropout and attempt to turn on the top MOSFET
continuously. A dropout detector detects this condition
and forces the top MOSFET to turn off for about 400ns
every 10th cycle to recharge the bootstrap capacitor, C_B.
The main control loop is shut down by pulling Pin 1 (RUN/
SS) 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, I_TH is
gradually released allowing normal operation to resume.
When the RUN/SS pin is low, all LTC1709 functions are
shut down. If V_OUT has not reached 70% of its nominal
value when C_SS has charged to 4.1V, an overcurrent
latchoff can be invoked as described in the Applications
Information section.
Low Current Operation
The LTC1709 operates in a continuous, PWM control
mode. The resulting operation at low output currents
optimizes transient response at the expense of substantial
negative inductor current during the latter part of the
period. The level of ripple current is determined by the
inductor value, input voltage, output voltage and fre-
quency of operation.
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.
Input capacitance ESR requirements and efficiency losses
are substantially reduced because the peak current drawn
from the input capacitor is effectively divided by two and
power loss is proportional to the RMS current squared. A
two stage, single output voltage implementation can
reduce input path power loss by 75% and radically reduce
the required RMS current rating of the input capacitor(s).
INTV_CC/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
of the IC circuitry is derived from INTV_CC. When the
EXTV_CC pin is left open, an internal 5V low dropout
regulator supplies INTV_CC power. If the EXTV_CC pin is
taken above 4.7V, the 5V regulator is turned off and an
internal switch is turned on connecting EXTV_CC to INTV_CC.
This allows the INTV_CC power to be derived from a high
efficiency external source such as the output of the regu-
lator itself or a secondary winding, as described in the
Applications Information section. An external Schottky
diode can be used to minimize the voltage drop from
EXTV_CC to INTV_CC in applications requiring greater than
the specified INTV_CC current. Voltages up to 7V can be
applied to EXTV_CC for additional gate drive capability.
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