ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 29

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the IC. These items are also illustrated graphically in the
layout diagram of Figure 11. Figure 12 illustrates the current
waveforms present in the various branches of the 2-phase
synchronous regulators operating in the continuous mode.
Check the following in your layout:
1. Are the top N-channel MOSFETs MTOP1 and MTOP2
2. Are the signal and power grounds kept separate? The
3. Do the LTC3858-2 V
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located within 1cm of each other with a common drain
connection at C
decoupling for the two channels as it can cause a large
resonant loop.
combined IC signal ground pin and the ground return
of C
minals. The path formed by the top N-channel MOSFET,
Schottky diode and the C
leads and PC trace lengths. The output capacitor (–)
terminals should be connected as close as possible
to the (–) terminals of the input capacitor by placing
the capacitors next to each other and away from the
Schottky loop described above.
the (+) terminals of C
connected between the (+) terminal of C
ground. The feedback resistor connections should not
be along the high current input feeds from the input
capacitor(s).
INTVCC
must return to the combined C
IN
? Do not attempt to split the input
FB
OUT
pins’ resistive dividers connect to
? The resistive divider must be
IN
capacitor should have short
OUT
OUT
and signal
(–) ter-
4. Are the SENSE
5. Is the INTV
6. Keep the switching nodes (SW1, SW2), top gate nodes
7. Use a modified “star ground” technique: a low imped-
minimum PC trace spacing? The filter capacitor between
SENSE
to the IC. Ensure accurate current sensing with Kelvin
connections at the SENSE resistor.
to the IC, between the INTV
pins? This capacitor carries the MOSFET drivers’ cur-
rent peaks. An additional 1μF ceramic capacitor placed
immediately next to the INTV
improve noise performance substantially.
(TG1, TG2), and boost nodes (BOOST1, BOOST2) away
from sensitive small-signal nodes, especially from the
opposites channel’s voltage and current sensing feed-
back pins. All of these nodes have very large and fast
moving signals and therefore should be kept on the
“output side” of the LTC3858-2 and occupy minimum
PC trace area.
ance, large copper area central grounding point on
the same side of the PC board as the input and output
capacitors with tie-ins for the bottom of the INTV
decoupling capacitor, the bottom of the voltage feedback
resistive divider and the SGND pin of the IC.
+
and SENSE
CC
–
decoupling capacitor connected close
and SENSE
–
should be as close as possible
+
CC
CC
leads routed together with
and PGND pins can help
and the power ground
LTC3858-2
29
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CC