LP3971_16 Datasheet, PDF (44/52 Pages) Texas Instruments – Power Management Unit for Advanced Application Processors

LP3971
SNVS432V – JANUARY 2006 – REVISED MAY 2013
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V_PP-ESR = (2 * I_RIPPLE) * R_ESR
(5)
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 can be expressed as follows
V_PP-RMS = VPP-C2 + VPP-ESR2
(6)
Note that the output voltage ripple is dependent on the inductor 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.
Model
GRM21BR60J106K
JMK212BJ106K
C2012X5R0J106K
Table 3. Suggested Capacitor and Their Suppliers
Type
Ceramic, X5R
Ceramic, X5R
Ceramic, X5R
Vendor
Murata
Taiyo-Yuden
TDK
Voltage
6.3V
6.3V
6.3V
Case Size
Inch (mm)
0805 (2012)
0805 (2012)
0805 (2012)
Buck Output Ripple Management
If V_IN and I_LOAD increase, the output ripple associated with the Buck Regulators also increases. The figure below
shows the safe operating area. To ensure operation in the area of concern it is recommended that the system
designer circumvents the output ripple issues to install schottky diodes on the Bucks(s) that are expected to
perform under these extreme corner conditions.
(Schottky diodes are recommended to reduce the output ripple, if system requirements include this shaded area
of operation. V_IN > 5.1V and I_LOAD > 1.24)
5.5
5.0
4.5
4.0
3.5
3.0
0
0.5
1.0
1.5
LOAD CURRENT (A)
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.
Good layout for the converters can be implemented by following a few simple design rules.
1. Place the converters, 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. Special care must be given to place the input filter capacitor very close to the V_IN
and GND pin.
2. 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 converter 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 converter by the inductor to the output filter capacitor and then back through
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