Datasheet

V
PP-RMS
= V
PP-C
2
+ V
PP-ESR
2
V
PP-C
=
4*f*C
I
RIPPLE
LM3691
SNVS506I –MAY 2008–REVISED MAY 2013 www.ti.com
The output voltage ripple is caused by the charging and discharging of the output capacitor and also due to its
R
ESR
and can be calculated as:
Voltage peak-to-peak ripple due to capacitance =
(2)
Voltage peak-to-peak ripple due to ESR =
V
PP-ESR
= (2 * I
RIPPLE
) * R
ESR
(3)
Because these two components are out of phase the rms value can be used to get an approximate value of
peak-to-peak ripple.
Voltage peak-to-peak ripple, root mean squared =
(4)
Note that the output voltage ripple is dependent on the current ripple and the equivalent series resistance of the
output capacitor (R
ESR
). The R
ESR
is frequency dependent (as well as temperature dependent); make sure the
value used for calculations is at the switching frequency of the part.
Table 2 lists suggested capacitors and suppliers.
Table 1. Suggested Inductors and Their Suppliers
Model Vendor Dimensions LxWxH (mm) D.C.R (mΩ)
LQM2HPN1R0MG0 Murata 2.5 x 2.0 x 1.0 55
MLP2520S1R0L TDK 2.5 x 2.0 x 1.0 60
KSLI252010BG1R0 HItachi Metals 2.5 x 2.0 x 1.0 80
MIPSZ2012D1R0 FDK 2.0 x 1.25 x 1.0 90
Table 2. Suggested Capacitors and Their Suppliers
Case Size
Model Type Vendor Voltage Rating (V)
Inch (mm)
4.7 µF for C
IN
and C
OUT
C1608X5R0J475K Ceramic TDK 6.3 0603 (1608)
C1608X5R1A475K Ceramic TDK 10.0 0603 (1608)
PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance
of a DC-DC converter and surrounding circuitry by contributing to EMI, ground bounce, and resistive voltage loss
in the traces. These can send erroneous signals to the DC-DC converter IC, resulting in poor regulation or
instability. In particular parasitic inductance from extra-long PCB trace lengths can cause additional noise
voltages through L*di/dt that adversely affect the DC-DC converter IC circuitry. Good layout for the LM3691 can
be implemented by following a few simple design rules.
1. Place the inductor and filter capacitors close together and make the traces short. The traces between these
components carry relatively high switching currents and act as antennas. Following this rule reduces radiated
noise.
2. Place the capacitors and inductor close to the LM3691. Place the C
IN
capacitor as close as possible to the
V
IN
and GND pads. Place the C
OUT
capacitor as close as possible to the VOUT and GND connections.
3. Arrange the components so that the switching current loops curl in the same direction. During the first half of
each cycle, current flows from the input filter capacitor, through the buck and inductor to the output filter
capacitor and back through ground, forming a current loop. In the second half of each cycle, current is pulled
up from ground, through the buck by the inductor, to the output filter capacitor and then back through ground,
forming a second current loop. Routing these loops so the current curls in the same direction prevents
magnetic field reversal between the two half-cycles and reduces radiated noise.
4. Connect the ground pins of the buck and filter capacitors together using generous component-side copper fill
as a pseudo-ground plane. Connect this to the ground-plane (if one is used) with several vias. This reduces
16 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM3691