LTC3785
17
3785fc
applicaTions inForMaTion
PC BOARD LAYOUT CHECKLIST
The basic PC board layout requires a dedicated ground
plane layer. Also, for high current, a multilayer board
provides heat sinking for power components.
•
The ground plane layer should not have any traces and
it should be as close as possible to the layer with power
MOSFETs.
• Place C
IN
, switch A, switch B and D1 in one compact
area. Place C
OUT
, switch C, switch D and D2 in one
compact area.
• U
se immediate vias to connect the components (includ-
ing the LTC3785’s GND/PGND pin) to the ground plane.
Use several large vias for each power component.
•
Use planes for V
IN
and V
OUT
to maintain good voltage
filtering and to keep power losses low.
• F
lood all unused areas on all layers with copper. Flooding
with copper will reduce the temperature rise of power
components. Connect the copper areas to any DC net
(V
IN
or GND). When laying out the printed circuit board,
the following checklist should be used to ensure proper
operation of the LTC3785.
•
S
egregate the signal and power grounds. All small-signal
components should return to the GND pin at one point.
The sources of switch B and switch C should also con-
nect to one point at the GND of the IC.
• Place
switch B and switch C as close to the controller
as possible, keeping the PGND, BG and SW traces
short.
• Keep the high dV/dT SW1, SW2, V
BST1
, V
BST2
, TG1 and
TG2 nodes away from sensitive small-signal nodes.
• The
path formed by switch A, switch B, D1 and the C
IN
capacitor should have short leads and PC trace lengths.
The path formed by switch C, switch D, D2 and the
C
OUT
capacitor also should have short leads and PC
trace lengths.
•
The output capacitor (–) terminals should be connected
as close as possible to the (–) terminals of the input
capacitor.
• Connect the V
CC
decoupling capacitor C
VCC
closely to
the V
CC
and PGND pins.
• Connect the top driver boost capacitor C
A
closely to
the V
BST1
and SW1 pins. Connect the top driver boost
capacitor C
B
closely to the V
BST2
and SW2 pins.
• Connect the input capacitors C
IN
and output capaci-
tors C
OUT
close to the power MOSFETs. These capaci-
tors carry the MOSFET AC current in boost and buck
mode.
•
Connect
FB and V
SENSE
pin resistive dividers to the (+)
terminals of C
OUT
and signal ground. If a small V
SENSE
decoupling capacitor is used, it should be as close as
possible to the LTC3785 GND pin.
• Route
I
SVIN
and I
SSW1
leads together with minimum PC
trace spacing. Ensure accurate current sensing with Kel-
vin connections across MOSFET A or sense resistor.
•
Route
I
SVOUT
and I
SSW2
leads together with minimum
PC trace spacing. Ensure accurate current sensing
with Kelvin connections across MOSFET D or sense
resistor.
•
Connect the feedback network close to IC, between the
V
C
and FB pins.
