lt3845a Linear Technology Corporation, lt3845a Datasheet - Page 20

• Manufacturer Part Number: lt3845a
• Description: Lt3845a - High Voltage Synchronous Current Mode Step-down Controller With Adjustable Operating Frequency
• Manufacturer: Linear Technology Corporation

LT3845A
Effective grounding techniques are critical for successful
DC/DC converter layouts. Orient power path components
such that current paths in the ground plane do not cross
through signal ground areas. Signal ground refers to the
Exposed Pad on the backside of the LT3845A IC in the
TSSOP package. SGND is referenced to the (–) terminal of
the V
voltage feedback reference. Power ground currents are
controlled on the LT3845A via the PGND pin, and this
ground references the high current synchronous switch
drive components, as well as the local V
important to keep PGND and SGND voltages consistent
with each other, so separating these grounds with thin
traces is not recommended. When the synchronous
FET is turned on, gate drive surge currents return to the
LT3845A PGND pin from the FET source. The BOOST supply
refresh surge currents also return through this same path.
The synchronous FET must be oriented such that these
PGND return currents do not corrupt the SGND reference.
Problems caused by the PGND return path are generally
recognized during heavy load conditions, and are typically
evidenced as multiple switch pulses occurring during a
single switch cycle. This behavior indicates that SGND is
being corrupted and grounding should be improved. SGND
corruption can often be eliminated, however, by adding a
small capacitor (100pF to 200pF) across the synchronous
switch FET from drain to source.
The high di/dt loop formed by the switch MOSFETs and
the input capacitor (C
to minimize high frequency noise and voltage stress from
inductive ringing. Surface mount components are preferred
to reduce parasitic inductances from component leads.
Connect the drain of the main switch MOSFET directly to
the (+) plate of C
chronous switch MOSFET directly to the (–) terminal of
C
MOSFETs. Switch path currents can be controlled by
orienting switch FETs, the switched inductor, and input
and output decoupling capacitors in close proximity to
each other.
APPLICATIONS INFORMATION
20
IN
. This capacitor provides the AC current to the switch
OUT
decoupling capacitor and is used as the converter
IN
, and connect the source of the syn-
IN
) should have short wide traces
CC
supply. It is
Locate the V
proximity to the IC. These capacitors carry the MOSFET
drivers’ high peak currents. Locate the small-signal
components away from high frequency switching nodes
(BOOST, SW, TG, V
oriented on the left side of the LT3845A, while high current
switching nodes are oriented on the right side of the IC
to simplify layout. This also helps prevent corruption of
the SGND reference.
Connect the V
independent of any other nodes, such as the SENSE
The feedback resistors should be connected between
the (+) and (–) terminals of the output capacitor (C
Locate the feedback resistors in close proximity to the
LT3845A to minimize the length of the high impedance
V
The SENSE
and kept as short as possible.
The LT3845A TSSOP package has been designed to
efficiently remove heat from the IC via the Exposed Pad on
the backside of the package. The Exposed Pad is soldered
to a copper footprint on the PCB. This footprint should be
made as large as possible to reduce the thermal resistance
of the IC case to ambient air. See Figure 5 for placement
of power path components.
FB
Figure 5. Orientation of Components Isolates Power Path and
PGND Currents, Preventing Corruption of SGND Reference
COMPONENTS
node.
REFERRED
SGND
–
CC
and SENSE
and BOOST decoupling capacitors in close
FB
SGND
pin directly to the feedback resistors
LT3845A
CC
BOOST
PGND
and BG). Small-signal nodes are
+
V
SW
CC
traces should be routed together
+
V
V
OUT
IN
+
I
SENSE
TG
BG
3845A AI03
SW
–
OUT
3845afa
pin.
).