ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 27

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

APPLICATIONS INFORMATION
The duty cycle percentage should be maintained from cycle
to cycle in a well-designed, low noise PCB implementation.
Variation in the duty cycle at a subharmonic rate can suggest
noise pickup at the current or voltage sensing inputs or
inadequate loop compensation. Overcompensation of the
loop can be used to tame a poor PC layout if regulator
bandwidth optimization is not required. Only after each
controller is checked for its individual performance should
both controllers be turned on at the same time. A particularly
diffi cult region of operation is when one controller channel
is nearing its current comparator trip point when the other
channel is turning on its top MOSFET. This occurs around
50% duty cycle on either channel due to the phasing of the
internal clocks and may cause minor duty cycle jitter.
Reduce V
of the regulator in dropout. Check the operation of the
undervoltage lockout circuit by further lowering V
monitoring the outputs to verify operation.
V
OUT1
3.3V
5A
L1, L2: COILTRONICS HCP0703
M1, M2: VISHAY SILICONIX Si4816BDY
C
OUT1
IN
3.3μH
C
100μF
X2
OUT1
L1
6.19k
, C
from its nominal level to verify operation
63.4k
1%
33pF
1%
OUT2
: TAIYO YUDEN JMK325BJ107MM
20k
1%
M1
1.33k
1%
4.7μF
Figure 14. High Effi ciency Dual 500kHz 3.3V/1.8V Step-Down Converter
4.75k
1%
1800pF
0.1μF
0.1μF
100pF
2.2Ω
0.1μF
D3
TG1
BOOST1
SW1
BG1
MODE/PLLIN
SENSE1
SENSE1
RUN1
V
I
V
TK/SS1
TH1
FB1
IN
PGOOD
IN
+
–
LTC3850-2
while
1μF
SGND
EXTV
FREQ/PLLFLTR
CC
SENSE2
SENSE2
BOOST2
TK/SS2
INTV
PGND
RUN2
Investigate whether any problems exist only at higher out-
put currents or only at higher input voltages. If problems
coincide with high input voltages and low output currents,
look for capacitive coupling between the BOOST, SW, TG,
and possibly BG connections and the sensitive voltage
and current pins. The capacitor placed across the current
sensing pins needs to be placed immediately adjacent to
the pins of the IC. This capacitor helps to minimize the
effects of differential noise injection due to high frequency
capacitive coupling. If problems are encountered with
high current output loading at lower input voltages, look
for inductive coupling between C
MOSFET components to the sensitive current and voltage
sensing traces. In addition, investigate common ground
path voltage pickup between these components and the
SGND pin of the IC.
SW2
V
TG2
BG2
I
TH2
FB2
CC
+
–
0.1μF
D4
10k, 1%
3.16k
1%
0.1μF
0.1μF
5.49k
1%
2200pF
1.5k
1%
100pF
M2
IN
20k
1%
, Schottky and the top
LTC3850-2
4.12k
1%
25.5k
33pF
22μF
50V
2.2μH
1%
L2
38502 F14
V
7V TO
20V
IN
C
100μF
X2
27
V
1.8V
5A
OUT2
OUT2
38502f