LTC3614_15 Datasheet, PDF (25/30 Pages) Linear Technology – 4A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3614
Applications Information
Assuming worst-case conditions of V_IN = 2V_OUT, C_IN should
be selected for a maximum current rating of:
I_RMS
=
4A
•
1.8V
3.6V
•


3.6V
1.8V
–
1^{}
=
2A_RMS
Decoupling PV_IN with four 10µF to 22µF capacitors is
adequate for most applications.
If we set R2 = 196k, the value of R1 can now be determined
by solving the following equation.
R1
=
196k
•
 1.8V
 0.6V
−
1^{}
A value of 392k will be selected for R1.
Finally, define the soft start-up time choosing the proper
value for the capacitor and the resistor connected to
TRACK/SS. If we set minimum t_SS = 5ms and a resistor
of 2MΩ, the following equation can be solved with the
maximum SV_IN = 5.5V :
C_SS
=
2MΩ
•
5ms
In^{}
5.5V
5.5V – 0.6V


=
21.6nF
The standard value of 22nF guarantees the minimum soft-
start up time of 5ms.
Figure 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 LTC3614:
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 pin close to the LTC3614.
2. Connect the (+) terminal of the input capacitor(s), C_IN,
as close as possible to the PV_IN pin, and the (–) terminal
as close as possible to the exposed pad, PGND. This
capacitor provides the AC current into the internal power
MOSFETs.
3. Keep the switching node, SW, away from all sensitive
small-signal nodes.
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_FB pin directly to the feedback resistors.
The resistor divider must be connected between V_OUT
and SGND.
For more information www.linear.com/LTC3614
3614fc
25