Datasheet

LTC4268-1
42
42681fc
In order to minimize switching noise and improve output
load regulation, connect the GND pin of the LTC4268-1
directly to the ground terminal of the V
CC
decoupling
capacitor, the bottom terminal of the current sense resistor
and the ground terminal of the input capacitor, using a
ground plane with multiple vias. Place the V
CC
capacitor
immediately adjacent to the V
CC
and GND pins on the IC
package. This capacitor carries high di/dt MOSFET gate
drive currents. Use a low ESR ceramic capacitor. Take care
in PCB layout to keep the traces that conduct high switching
currents short, wide and with minimal overall loop area.
These are typically the traces associated with the switches.
This reduces the parasitic inductance and also minimizes
magnetic field radiation. Figure 21 outlines the critical paths.
Keep electric field radiation low by minimizing the length
and area of traces (keep stray capacitances low). The drain
of the primary side MOSFET is the worst offender in this
category. Always use a ground plane under the switcher
circuitry to prevent coupling between PCB planes. Check
that the maximum BV
DSS
ratings of the MOSFETs are not
exceeded due to inductive ringing. This is done by viewing
the MOSFET node voltages with an oscilloscope. If it is
breaking
down either choose a higher voltage device, add
a snubber or specify an avalanche-rated MOSFET.
Place the small-signal components away from high
frequency switching nodes. This allows the use of a pseudo-
Kelvin connection for the signal ground, where high di/
dt gate driver currents flow out of the IC ground pin in
one direction (to the bottom plate of the V
CC
decoupling
capacitor) and small-signal currents flow in the other
direction. Keep the trace from the feedback divider tap
to the FB pin short to preclude inadvertent pickup. For
applications with multiple switching power converters
connected to the same input supply, make sure that the
input filter capacitor for the LTC4268-1 is not shared with
other converters. AC input current from another converter
could cause substantial input voltage ripple and this could
interfere with the LTC4268-1 operation. A few inches of PC
trace or wire (L @100nH) between the C
IN
of the LTC4268-1
and the actual source V
IN
is sufficient to prevent current
sharing problems.
T2
T1
C
R
C
VIN
MS
MP
GATE
TURN-ON
GATE
TURN-ON
R
SENSE
• •
C
VCC
SG
V
CC
PG
V
CC
V
CC
V
CC
V
IN
GATE
TURN-OFF
GATE
TURN-OFF
Q4
Q3
C
OUT
42681 F21
OUT
•
•
•
+
+
+
Figure 21. Layout Critical High Current Paths
applicaTions inForMaTion