Datasheet

LT3825
26
3525fe
PC Board Layout Considerations
In order to minimize switching noise and improve output
load regulation, connect the GND pin of the LT3825 directly
to the ground terminal of the V
CC
decoupling capacitor,
the bottom terminal of the current sense resistor, the
ground terminal of the input capacitor, and the ground
plane (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 9 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 fre-
quency 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 capaci-
tor) 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 LT3825 is not shared with
other
converters. AC input current from another converter
could cause substantial input voltage ripple and this could
interfere with the LT3825 operation. A few inches of PC
trace or wire (L @ 100nH) between the C
IN
of the LT3825
and the actual source V
IN
is sufficient to prevent current
sharing problems.
APPLICATIONS INFORMATION
T2
T1
C
R
C
VIN
MS
MP
GATE
TURN-ON
GATE
TURN-ON
R
SENSE
• •
C
VCC
SG
V
CC
GND
LT3825
PG
V
CC
V
CC
V
CC
V
IN
GND
LT3825
GATE
TURN-OFF
GATE
TURN-OFF
Q4
Q3
C
OUT
3825 F09
OUT
•
•
•
Figure 9. High Current Paths