Datasheet

LTC3615/LTC3615-1
26
3615fb
For more information www.linear.com/LTC3615
Decoupling the PV
IN
with two 47µF capacitors is adequate
for most applications.
Finally, it is possible to define the soft-start up time choos
-
ing the proper value for the capacitor and the resistor
connected to TRACK/SS pin. If one sets minimum T
SS
=
5ms and a resistor of 4.7M, the following equation can
be solved with the maximum SV
IN
= 5.5V:
C
SS
=
5ms
4.7M • In
5.5V
5.5V – 0.6V
⎛
⎝
⎜
⎞
⎠
⎟
= 9.2nF
The standard value of 10nF and 4.7M guarantees the
minimum soft-start time of 5ms. In Figure 3, channel 1
shows the schematic for this design example.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3615:
1. A ground plane is recommended. If a ground plane
layer is not
used, the signal and power grounds should
be segregated with all small signal components returning
to the SGND pin at one point which is then connected to
the PGND node at the exposed pad close to the LTC3615
2. Connect the (+) terminal of the input capacitors, C
IN
,
as close as possible to the PV
INx
pins, and the (–) ter-
minal as close as possible to the exposed pad PGND.
This
capacitor provides the AC current into the internal
power MOSFETs.
3. Keep the switching nodes, SWx, away from all sensitive
small signal nodes FBx, ITHx, RTSYNC, SRLIM.
4. Flood all unused areas on all layers with copper. Flood
-
ing with copper will reduce the temperature rise of
power components. Connect the copper areas to PGND
(exposed pad) for best performance.
5. Connect the V
FBx
pins directly to the feedback resis-
tors. The resistor divider must be connected between
V
OUTx
and SGND.
applicaTions inForMaTion
DDR Memory Termination
FB1
MODE
LTC3615
3615 TA03a
SGND
PGND
RUN1
TRACK/SS1
ITH1
PHASE
RUN2
TRACK/SS2
PGOOD2
ITH2
PGOOD1
R
T
/SYNC
SRLIM
V
IN
3.3V
SV
IN
C
IN1
47µF
C
IN2
47µF
C
IN3
1µF
L1
0.47µH
C
OUT1
47µF
V
DDQ
1.8V/3A
R1
121k
R2
60.4k
R7
15k
C4
1000pF
R10
15k
C2
1000pF
R3
150k
R4
49.9k
FB2
L2
0.47µH
C
OUT2
47µF
R5
49.9k
R6
49.9k
C1
10pF
C3
10pF
R9
226k
R8
174k
V
TT
0.9V
3A/–1.5A
(2×) SW1
(2×) PV
IN1
(2×) PV
IN2
(2×) SW2
Ratiometric Start-Up
3615 TA03b
500mV/
DIV
500µs/DIV
V
DD
V
TT
Typical applicaTions
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