lt3845ife-trpbf Linear Technology Corporation, lt3845ife-trpbf Datasheet - Page 18

• Manufacturer Part Number: lt3845ife-trpbf
• Description: High Voltage Synchronous Current Mode Step-down Controller With Adjustable Operating Frequency
• Manufacturer: Linear Technology Corporation

LT3845
APPLICATIONS INFORMATION
FET carry PGND currents. SGND originates at the negative
terminal of the V
signal reference for the LT3845.
Don’t be tempted to run small traces to separate ground
paths. A good ground plane is important as always,
but PGND referred bypass elements must be oriented
such that transient currents in these return paths do not
corrupt the SGND reference.
During the dead-time between switch conduction, the
body diode of the synchronous FET conducts inductor
current. Commutating this diode requires a signifi cant
charge contribution from the main switch. At the instant
the body diode commutates, a current discontinuity is
created and parasitic inductance causes the switch node
to fl y up in response to this discontinuity. High currents
and excessive parasitic inductance can generate ex-
tremely fast dV/dt rise times. This phenomenon can cause
avalanche breakdown in the synchronous FET body di-
ode, signifi cant inductive overshoot on the switch node,
and shoot-through currents via parasitic turn-on of the
synchronous FET. Layout practices and component ori-
entations that minimize parasitic inductance on this node
is critical for reducing these effects.
Ringing waveforms in a converter circuit can lead to device
failure, excessive EMI, or instability. In many cases, you
can damp a ringing waveform with a series RC network
across the offending device. In LT3845 applications, any
ringing will typically occur on the switch node, which
can usually be reduced by placing a snubber across the
synchronous FET. Use of a snubber network, however,
should be considered a last resort. Effective layout practices
typically reduce ringing and overshoot, and will eliminate
the need for such solutions.
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 LT3845 IC. SGND
is referenced to the (–) terminal of the V
capacitor and is used as the converter voltage feedback
reference. Power ground currents are controlled on the
LT3845 via the PGND pin, and this ground references
the high current synchronous switch drive components,
18
OUT
bypass capacitor, and is the small
OUT
decoupling
as well as the local V
PGND and SGND voltages consistent with each other, so
separating these grounds with thin traces is not recom-
mended. When the synchronous FET is turned on, gate
drive surge currents return to the LT3845 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.
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 LT3845, 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
IN
. This capacitor provides the AC current to the switch
CC
and BOOST decoupling capacitors in close
FB
IN
pin directly to the feedback resistors
, and connect the source of the syn-
CC
CC
IN
and BG). Small-signal nodes are
) should have short wide traces
supply. It is important to keep
–
OUT
3845fc
pin.
).