Datasheet

LTC3838-1
14
38381f
For more information www.linear.com/3838-1
OPERATION
(Refer to Functional Diagram)
Main Control Loop
The LTC3838-1 is a controlled on-time, valley current
mode step-down DC/DC dual controller with two channels
operating out of phase. Each channel drives both main
and synchronous N-channel MOSFETs. The two channels
can be either configured to two independently regulated
outputs, or combined into a single output.
The top MOSFET is turned on for
a time interval determined
by a one-shot timer. The duration of the one-shot timer is
controlled to maintain a fixed switching frequency. As the
top MOSFET is turned off, the bottom MOSFET is turned
on after a small delay. The delay, or dead time, is to avoid
both top and bottom MOSFETs being on at the same time,
causing shoot-through current from V
IN
directly to power
ground. The next switching cycle is initiated when the cur-
rent comparator, I
CMP
, senses that inductor current falls
below the trip level set by voltages at the ITH and V
RNG
pins. The bottom MOSFET is turned off immediately and
the top MOSFET on again, restarting the one-shot timer
and repeating the cycle. In order to avoid shoot-through
current, there is also a
small dead-time delay before the
top MOSFET turns on. At this moment, the inductor cur-
rent hits its “valley” and starts to rise again.
Inductor current is determined by sensing the voltage
between SENSE
+
and SENSE
–
, either by using an explicit
resistor connected in series with the inductor or by implicitly
sensing the inductor’s DC resistive (DCR) voltage drop
through an RC filter connected
across the inductor. The
trip level of the current comparator, I
CMP
, is proportional
to the voltage at the ITH pin, with a zero-current threshold
corresponding to an ITH voltage of around 0.8V.
The error amplifier (EA) adjusts this ITH voltage by
comparing the feedback signal to the internal reference
voltage. Output voltage is regulated so that the feedback
voltage is equal to the internal reference. If the load current
increases/decreases, it causes a momentary drop/rise
in
the differential feedback voltage relative to the reference.
The EA then moves ITH voltage, or inductor valley current
setpoint, higher/lower until the average inductor current
again matches the load current, so that the output voltage
comes back to the regulated voltage.
The LTC3838-1 features a detect transient (DTR) pin to
detect “load-release”, or a transient where the load current
suddenly drops, by monitoring
the first derivative of the
ITH voltage. When detected, the bottom gate (BG) is turned
off and inductor current flows through the body diode in
the bottom MOSFET, allowing the SW node voltage to
drop below PGND by the body diode’s forward-conduction
voltage. This creates a more negative differential voltage
(V
SW
– V
OUT
) across the inductor, allowing the inductor
current to drop faster to zero
, thus creating less overshoot
on V
OUT
. See Load-Release Transient Detection in Applica-
tions Information for details.
Differential Output Sensing
Both channels of this dual controller have differential output
voltage sensing. The output voltage is resistively divided
externally to create a feedback voltage for the controller.
As shown in the Functional Diagram, channel 1 uses an
external 2-resistor voltage divider, and an internal unity-
gain difference amplifier (DIFFAMP
) that converts the dif-
ferential feedback signal to a single-ended internal feedback
voltage V
FB1
= V
OUTSENSE1
+
– V
OUTSENSE1
–
with respect to
SGND. With the external resistor divider, V
OUT1
+
– V
OUT1
–
=
V
FB1
• (R
FB1
+ R
FB2
)/R
FB1
. Channel 2 has a unique feedback
amplifier that produces V
FB2
= 2 • V
DFB2
+
– V
DFB2
–
. Its
external feedback network requires a third resistor
tied
to local ground (SGND). The third resistor must have a
value equal to the parallel value of the two voltage divider
resistors R
DFB1
and R
DFB2
so that V
OUT2
+
– V
OUT2
–
=
V
FB2
• (R
DFB1
+ R
DFB2
)/R
DFB1
.
Both channels adjust outputs through system feedback
loops so that these internally-generated single-ended
feedback voltages (V
FB1,2
in the Functional Diagram)
are
equal to the internal 0.6V reference voltages when in
regulation. Therefore, the differential V
OUT1
is regulated
to 0.6V • (R
FB1
+ R
FB2
)/R
FB1
, and the differential V
OUT2
is
regulated to 0.6V • (R
DFB1
+ R
DFB2
)/R
DFB1
. Such schemes
eliminate any ground offsets between local ground and
remote output ground, resulting in a more accurate output
voltage. Channel 1 allows remote output ground to deviate
as much as ±500mV, and channel 2 allows as much as
±200mV, with respect to local ground (SGND).