LTC3633A-2/LTC3633A-3
11
3633a23fb
For more information www.linear.com/LTC3633A-2
OPERATION
The LTC3633A-2 is a dual-channel, current mode monolithic
step down regulator capable of providing 3A of output
current from each channel. Its unique controlled on-time
architecture allows extremely low step-down ratios while
maintaining a constant switching frequency. Each channel
is enabled by raising the voltage on the RUN pin above
1.22V nominally.
The LTC3633A-2 has a V
ON
sense range of 0.6V to 6V, while
the LTC3633A-3 has a V
ON
sense range of 1.5V to 12V. The
following table highlights the difference between the parts
in the 3633A family. Consult the LTC3633A/LTC3633A-1
data sheet for more details on specific characteristics of
those products.
Table 1. LTC3633A Family Features
PART
NUMBER
OUTPUT
VOLTAGE
SENSE RANGE
SV
IN
INPUT
V2P5
OUTPUT
LTC3633 PIN
COMPATIBLE
LTC3633A 0.6V TO 6V NO YES YES
LTC3633A-1 1.5V TO 12V NO YES YES
LTC3633A-2 0.6V TO 6V YES NO NO
LTC3633A-3 1.5V TO 12V YES NO NO
Main Control Loop
In normal operation, the internal top power MOSFET is
turned on for a fixed interval determined by a fixed one-shot
timer (“ON” signal in Block Diagram). When the top power
MOSFET turns off, the bottom power MOSFET turns on until
the current comparator I
CMP
trips, thus restarting the one
shot timer and initiating the next cycle. Inductor current is
measured by sensing the voltage drop across the SW and
PGND nodes of the bottom power MOSFET. The voltage on
the ITH pin sets the comparator threshold corresponding
to inductor valley current. The error amplifier EA adjusts
this ITH voltage by comparing an internal 0.6V reference to
the feedback signal V
FB
derived from the output voltage. If
the load current increases, it causes a drop in the feedback
voltage relative to the internal reference. The ITH voltage
then rises until the average inductor current matches that
of the load current.
The operating frequency is determined by the value of the
RT resistor, which programs the current for the internal os
-
cillator. An internal phase-locked loop servos the switching
regulator on-time to track the internal oscillator edge and
for
ce a constant switching frequency. A clock signal can be
applied to the MODE/SYNC pin to synchronize the switching
frequency to an external source. The regulator defaults to
forced continuous operation once the clock signal is applied.
At light load currents, the inductor current can drop to zero
and become negative. In Burst Mode operation, a current
reversal comparator (I
REV
) detects the negative inductor
current and shuts off the bottom power MOSFET, result-
ing in discontinuous operation and increased efficiency.
Both power MOSFET
s will remain off until the ITH voltage
rises above the zero current level to initiate another cycle.
During this time, the output capacitor supplies the load
current and the part is placed into a low current sleep
mode. Discontinuous mode operation is disabled by tying
the MODE/SYNC pin to ground, which forces continuous
synchronous operation regardless of output load current.
“Power Good” Status Output
The PGOOD open-drain output will be pulled low if the
regulator output exits a ±8% window around the regulation
point. This condition is released once regulation within a
±5% window is achieved. To prevent unwanted PGOOD
glitches during transients or dynamic V
OUT
changes, the
LTC3633A-2 PGOOD falling edge includes a filter time of
approximately 40µs.
PV
IN
Overvoltage Protection
In order to protect the internal power MOSFET devices
against transient input voltage spikes, the LTC3633A-2
constantly monitors each PV
IN
pin for an overvoltage
condition. When PV
IN
rises above 22.5V, the regulator
suspends operation by shutting off both power MOSFETs
on the corresponding channel. Once PV
IN
drops below
21.5V, the regulator immediately resumes normal opera-
tion. The regulator executes its soft-start function when
exiting an over
voltage condition.
Out-Of-Phase Operation
T
ying the PHMODE pin high sets the SW2 falling edge to
be 180° out of phase with the SW1 falling edge. There is
a significant advantage to running both channels out of
phase. When running the channels in phase, both top-side
MOSFETs are on simultaneously, causing large current
pulses to be drawn from the input capacitor and supply
at the same time.
