LTC3862 Datasheet, PDF (34/40 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
APPLICATIONS INFORMATION
For the bulk capacitance, which we assume contributes 2. In order to help dissipate the power from the MOSFETs
1% to the total output ripple, the minimum required
and diodes, keep the ground plane on the layers closest
capacitance is approximately:
to the power components. Use power planes for the
C_OUT
≥
I_O(MAX)
0.01• n • V_OUT
•
f
=
0.01•
2
•
5A
48V
•
300kHz
T
3.
MOSFETs and diodes in order to maximize the heat
spreading from these components into the PCB.
Place all power components in a tight area. This will
= 17.5μF
For this application, in order to obtain both low ESR
and an adequate ripple current rating (see Figure 23),
two 100μF, 63V aluminum electrolytic capacitors were
minimize the size of high current loops. The high di/dt
loops formed by the sense resistor, power MOSFET,
the boost diode and the output capacitor should be
kept as small as possible to avoid EMI.
connected in parallel with four 6.8μF, 50V ceramic ^{4. Orient the input and output capacitors and current}
capacitors. Figure 26 illustrates the switching wave-
sense resistors in a way that minimizes the distance
forms for this application circuit.
between the pads connected to the ground plane.
Keep the capacitors for INTV_CC, 3V8 and V_IN as close
as possible to LTC3862.
SW1
50V/DIV
I_L1
5A/DIV
SW2
50V/DIV
I_L2
5A/DIV
V_OUT
100mV/DIV
AC COUPLED
V_IN = 24V
V_OUT = 48V, 1.5A
2.5μs/DIV
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Figure 26. LTC3862 Switching Waveforms for Boost Converter
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the converter:
1. For lower power applications a 2-layer PC board is suf-
ficient. However, for higher power levels, a multilayer
PC board is recommended. Using a solid ground plane
and proper component placement under the circuit is
the easiest way to ensure that switching noise does
not affect the operation.
5. Place the INTV_CC decoupling capacitor as close as
possible to the INTV_CC and PGND pins, on the same
layer as the IC. A low ESR (X5R or better) 4.7μF to
10μF ceramic capacitor should be used.
6. Use a local via to ground plane for all pads that
connect to the ground. Use multiple vias for power
components.
7. Place the small-signal components away from high
frequency switching nodes on the board. The pinout
of the LTC3862 was carefully designed in order to
make component placement easy. All of the power
components can be placed on one side of the IC, away
from all of the small-signal components.
8. The exposed area on the bottom of the QFN package
is internally connected to PGND; however it should
not be used as the main path for high current flow.
9. The MOSFETs should also be placed on the same
layer of the board as the sense resistors. The MOSFET
source should connect to the sense resistor using a
short, wide PCB trace.
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