Datasheet

LTC3618
10
3618fc
For more information www.linear.com/LTC3618
operaTion
Main Control Loop
The LTC3618 is a dual monolithic step-down DC/DC con-
verter featuring
current-mode, constant-frequency opera-
tion. The r
egulated output voltage of the second step-down
converter is equal to VDDQIN • 0.5. An additional internal
amplifier provides a VTTR output equal to VDDQIN • 0.5,
which is capable of driving a ±10mA load.
During normal operation, the internal top power switch
(P-channel MOSFET) of each channel is turned on at
the beginning of its clock cycle. Current in the inductor
increases until the current comparator trips and turns
off the top power MOSFET. The peak inductor current at
which the current comparator shuts off is controlled by
the voltage on the ITH pin. The error amplifier adjusts
the voltage on the ITH pin by comparing the feedback
signals V
FBX
(derived from an external resistor divider on
the V
FB1
pin) with a reference (0.6V for V
DDQ
, VDDQIN •
0.5 for V
TT
). When the load current increases, it causes a
reduction in the feedback voltage relative to the reference.
The error amplifier raises the ITH voltage until the average
inductor current matches the new load current. Typical
voltage range for the ITH pin is from 0.55V
to 1.05V with
0.55V corresponding to zero current.
When
the top power MOSFET shuts off, the synchronous
power switch (N-channel MOSFET) turns on until either
the current limit is reached or the next clock cycle begins.
The bottom current limit is typically set at –4A for forced
continuous mode and 0A for pulse-skipping mode.
The operating frequency defaults to 2.25MHz when
RT is
connected to SV
IN
, or can be set by an external
resistor connected between the RT pin and ground, or by
a clock signal applied to the MODE/SYNC pin. The switch-
ing frequency
can be set from 400kHz to 4MHz (see the
Applications Information section).
Overvoltage and undervoltage comparators pull the PGOOD
output low if the output voltage varies more than ±8%
(typical) from the set point.
V
IN
Overvoltage Protection
In order to protect the internal power MOSFET devices
against transient voltage spikes, the LTC3618 constantly
monitors the V
IN
pin for an overvoltage condition. When
V
IN
rises above 6.5V, the regulator suspends operation
by shutting off the MOSFETs. The regulator executes its
soft-start when exiting an overvoltage condition.
MODE SELECTION
The MODE/SYNC pin is used to select one of two different
operating
modes for V
DDQ
. When the MODE/SYNC pin
is tied to SV
IN
, pulse-skipping mode is selected, when
it is tied to ground, forced continuous mode is selected
(Figure 1). V
TT
is always in forced continuous mode.
VTTR Voltage Buffer Output
An internal high accuracy op amp buffer generates
a VTTR pin voltage that is equal to VDDQIN • 0.5.
VTTR can source and sink up to 10mA and is stable
with a 0.1µF capacitor. Short circuit current limit is
set around 20mA to prevent damage to the op amp.
The VTTR output is also the reference voltage for V
TT
.
Therefore, large
transients on this pin will impact the
behavior at the V
TT
output.
Figure 1. V
DDQ
Modes of Operation
LTC3618
SV
IN
V
IN
MODE
SGND
0V
LTC3618
SV
IN
V
IN
MODE
SGND
0V
3618 F01
1a. Pulse-Skipping Mode 1b. Forced Continuous Mode