ltc1629-6 Linear Technology Corporation, ltc1629-6 Datasheet - Page 10

• Manufacturer Part Number: ltc1629-6
• Description: Polyphase, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

LTC1629-6
OPERATIO
Main Control Loop
The LTC1629-6 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
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 de-
crease in the EAIN pin voltage relative to the 0.6V refer-
ence, which in turn causes the I
the average inductor current matches the new load cur-
rent. 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
each off cycle through an external Schottky diode. When
V
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 1.2 A
current source to charge soft-start capacitor C
C
I
value. As C
leased allowing normal operation to resume. When the
RUN/SS pin is low, all LTC1629-6 functions are shut
down. If V
when C
be invoked as described in the Applications Information
section.
10
TH
IN
SS
voltage clamped at approximately 30% of its maximum
decreases to a voltage close to V
reaches 1.5V, the main control loop is enabled with the
SS
has charged to 4.1V, an overcurrent latchoff can
OUT
SS
has not reached 70% of its nominal value
continues to charge, I
TH
B
U
, which normally is recharged during
pin, which is the output of the error
(Refer to Functional Diagram)
TH
voltage to increase until
OUT
TH
, however, the loop
is gradually re-
SS
. When
Low Current Operation
The LTC1629-6 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.
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
Controller 2
CLKOUT
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).
PHASMD
180
GND
60
OPEN
180
90
INTV
240
120
CC
16296f