LTC3355_15 Datasheet, PDF (17/20 Pages) Linear Technology – 20V 1A Buck DC/DC with Integrated SCAP Charger and Backup Regulator

LTC3355
APPLICATIONS INFORMATION
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Large switched
currents flow in the V_IN, SW1, SW2 and paddle ground
pins, the buck catch diode, boost rectifier diode and the
input capacitor. The loop formed by these components
should be as small as possible. These components, along
with the inductors and output capacitor, should be placed
on the same side of the circuit board, and their connec-
tions should be made on that layer. All connections to
GND should be made at a common star ground point or
directly to a local, unbroken ground plane below these
components. SW1 and SW2 nodes should be laid out
carefully to avoid interference. Keep the FB, PFI, I_CHG,
I_BSTPK, V_CBST and CFB nodes small so that the ground
traces will shield them from the switching nodes. To
keep thermal resistance low, extend the ground plane as
much as possible and add thermal vias under and near the
paddle. Keep in mind that the thermal design must keep
the junctions of the LTC3355 below the specified absolute
maximum temperature.
High Temperature Considerations
The PCB must provide heat sinking to keep the LTC3355
cool. The exposed pad on the bottom of the package
may be soldered to a copper area which should be tied
to large copper layers below with thermal vias; these
layers will spread the heat dissipated by the LTC3355.
Place additional vias to reduce thermal resistance
further. With these steps, the thermal resistance from
the die (or junction) to ambient can be reduced to
θ_JA = 47°C/W or less. With 100 LFPM airflow, this resis-
tance can fall by another 25%.
The LTC3355 has two thermal circuits. The first thermal
circuit is operational when the buck and charger are enabled.
If the die temperature exceeds 110°C the charge current
will be reduced. When the LTC3355 is in boost mode the
high current thermal shutdown will turn the boost off when
the die temperature reaches 155°C. The high temperature
shutdown is active in all modes of operation.
TYPICAL APPLICATIONS
Tantalum Capacitor Charger and Ride-Through Backup Supply
R_S
V_IN
12V
1Ω
C_IN
C_VIN
C_CAP ^{5 VIN}
10µF
10µF
1µF _{6 VINM5}
4 ^VINS
R7
2.49M
1 PFI
R8
200k
10 PFOB
13 RSTB
9 CPGOOD
8 EN_CHG
SW1 ₇
PFO
BUCK _{VOUT 15}
LTC3355
BOOST
FB 2
V_{CAP 14}
SW2
16
17
CFB 11
L1
6.8µH
D1
R1
523k
V_OUT
5V
47µF 10mA
D2
R2
100k
L2 3.3µH
+
R3
1.05M
5V
1000µF
6.3V
TANT
3 MODE
INTV_CC
18
C1
I_CHG
12
R5
V_CBST 19
I_BSTPK
20
R6
R_C
154k
C_C
220pF
R4
200k
1µF 604k
1M
3355 TA02
For more information www.linear.com/LTC3355
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