LTC3729
10
3729fb
OPERATION
Main Control Loop
The LTC3729 uses a constant frequency, current mode
step‑down architecture. 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 differential amplifier, A1, produces a
signal equal to the differential 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 pin which 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 bootstrap
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.
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 LTC3729 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 LTC3729 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 frequency 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 250kHz to 550kHz 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.
The internal master oscillator runs at a frequency twelve
times that of each controller’s frequency. The PHASMD
pin determines the relative phases between the internal
controllers as well as the CLKOUT signal as shown in
Table 1. The phases tabulated are relative to zero phase
being defined as the rising edge of the top gate (TG1)
driver output of controller 1.
Table 1.
V
PHASMD
GND OPEN INTV
CC
Controller 2 180° 180° 240°
CLKOUT 60° 90° 120°
The CLKOUT signal can be used to synchronize additional
power stages in a multiphase power supply solution feeding
a single, high current output or separate outputs. Input
capacitance ESR requirements and efficiency losses are
substantially reduced because the peak current drawn from
the input capacitor is effectively divided by the number
of phases used 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).
(Refer to Functional Diagram)
