LTC3615-1_15 Datasheet, PDF (26/32 Pages) Linear Technology – Dual 4MHz, 3A Synchronous Step-Down DC/DC Converter

LTC3615/LTC3615-1
Applications Information
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
=
4.7M
•
In
5ms
⎛ 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.
Typical Applications
DDR Memory Termination
V_IN
3.3V
C_IN1
C_IN2
C_IN3
47µF
47µF
1µF
SV_IN (2×) PV_IN1 (2×) PV_IN2
L1
0.47µH
RUN1
(2×) SW1
V_DDQ
1.8V/3A
TRACK/SS1
R1
R3
C_OUT1
PGOOD1
121k 150k
47µF
R10
15k
C1
10pF
C2
1000pF
R8
174k
ITH1
FB1
R_T /SYNC _LTC3615
SRLIM
R2
60.4k
R4
49.9k
MODE
L2
R9
0.47µH
V_TT
226k
(2×) SW2
0.9V
PHASE
RUN2
FB2
TRACK/SS2
R5
49.9k
R6
3A/–1.5A
C_OUT2
47µF
PGOOD2
49.9k
500mV/
DIV
R7
15k
C3
10pF
ITH2 SGND PGND
3615 TA03a
C4
1000pF
Ratiometric Start-Up
V_DD
V_TT
500µs/DIV
3615 TA03b
26
For more information www.linear.com/LTC3615
3615fb