ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 25

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
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 12. Figure 13 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 M1 and M3 located within
2. Are the signal and power grounds kept separate? The
3. Do the LTC3850-2 V
4. Are the SENSE
1 cm of each other with a common drain connection at
C
two channels as it can cause a large resonant loop.
combined IC signal ground pin and the ground return
of C
terminals. The V
as possible. The path formed by the top N-channel
MOSFET, Schottky diode and the C
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
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).
minimum PC trace spacing? The fi lter capacitor between
SENSE
to the IC. Ensure accurate current sensing with Kelvin
connections at the sense resistor or inductor, whichever
is used for current sensing.
IN
? Do not attempt to split the input decoupling for the
INTVCC
+
and SENSE
must return to the combined C
+
and SENSE
FB
FB
and I
–
OUT
pins’ resistive dividers connect to
should be as close as possible
? The resistive divider must be
TH
–
traces should be as short
leads routed together with
IN
capacitor should
OUT
and signal
OUT
(–)
5. Is the INTV
6. Keep the switching nodes (SW1, SW2), top gate nodes
7. Use a modifi ed “star ground” technique: a low imped-
PC Board Layout Debugging
Start with one controller at a time. It is helpful to use a
DC-50MHz current probe to monitor the current in the
inductor while testing the circuit. Monitor the output
switching node (SW pin) to synchronize the oscilloscope
to the internal oscillator and probe the actual output voltage
as well. Check for proper performance over the operating
voltage and current range expected in the application. The
frequency of operation should be maintained over the input
voltage range down to dropout and until the output load
drops below the low current operation threshold—typically
10% of the maximum designed current level in Burst
Mode operation.
the IC, between the INTV
This capacitor carries the MOSFET drivers current peaks.
An additional 1μF ceramic capacitor placed immediately
next to the INTV
noise performance substantially.
(TG1, TG2), and boost nodes (BOOST1, BOOST2) away
from sensitive small-signal nodes, especially from the
opposite channel’s voltage and current sensing feedback
pins. All of these nodes have very large and fast moving
signals and therefore should be kept on the “output
side” of the LTC3850-2 and occupy minimum PC trace
area. If DCR sensing is used, place the top resistor
(Figure 2b, R1) close to the switching node.
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.
CC
decoupling capacitor connected close to
CC
and PGND pins can help improve
CC
and the power ground pins?
LTC3850-2
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CC