LP3971_16 Datasheet, PDF (43/52 Pages) Texas Instruments – Power Management Unit for Advanced Application Processors

LP3971
www.ti.com
SNVS432V – JANUARY 2006 – REVISED MAY 2013
• I_RIPPLE: Average to peak inductor current
• I_OUTMAX: Maximum load current (1500 mA)
• V_IN: Maximum input voltage in application
• L: Min inductor value including worst case tolerances (30% drop can be considered for method 1)
• f: Minimum switching frequency (1.6 MHz)
• V_OUT: Output voltage
Method 2
A more conservative and recommended approach is to choose an inductor that has saturation current rating
greater than the max current limit of TBD mA.
A 2.2 μH inductor with a saturation current rating of at least TBD mA is recommended for most applications. The
inductor’s resistance should be less than 0.3Ω for a good efficiency. Table 2 lists suggested inductors and
suppliers. For low-cost applications, an unshielded bobbin inductor could be considered. For noise critical
applications, a toroidal or shielded bobbin inductor should be used. A good practice is to lay out the board with
overlapping footprints of both types for design flexibility. This allows substitution of a low-noise shielded inductor,
in the event that noise from low-cost bobbin models is unacceptable.
Input Capacitor Selection
A ceramic input capacitor of 10 μF, 6.3V is sufficient for most applications. Place the input capacitor as close as
possible to the V_IN pin of the device. A larger value may be used for improved input voltage filtering. Use X7R or
X5R types, do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting
case sizes like 0805 and 0603. The input filter capacitor supplies current to the PFET switch of the converter in
the first half of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capacitor’s
low ESR provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select a
capacitor with sufficient ripple current rating. The input current ripple can be calculated as:
I_RMS = I_{OUTMAX *}
V_OUT
V_IN
* _{¨¨©§}^{1 -}
V_OUT
V_IN
r²
+
12
(V_IN - V_OUT) _* V_OUT
where r =
L _* f _* I_{OUTMAX *} VIN
(3)
The worst case is when V_IN = 2 * V_OUT
Model
FDSE0312-2R2M
DO1608C-222
Table 2. Suggested Inductors and Their Suppliers
Vendor
Toko
Coilcraft
Dimensions LxWxH (mm)
3.0 x 3.0 x 1.2
6.6 x 4.5 x 1.8
D.C.R (Typ)
160 mΩ
80 mΩ
Output Capacitor Selection
Use a 10 μF, 6.3V ceramic capacitor. Use X7R or X5R types, do not use Y5V. DC bias characteristics of ceramic
capacitors must be considered when selecting case sizes like 0805 and 0603. DC bias characteristics vary from
manufacturer to manufacturer and dc bias curves should be requested from them as part of the capacitor
selection process. The output filter capacitor smooths out current flow from the inductor to the load, helps
maintain a steady output voltage during transient load changes and reduces output voltage ripple. These
capacitors must be selected with sufficient capacitance and sufficiently low ESR to perform these functions.
The output voltage ripple is caused by the charging and discharging of the output capacitor and also due to its
ESR and can be calculated as:
I_RIPPLE
V_PP-C = _{4 * f * C}
(4)
Voltage peak-to-peak ripple due to ESR can be expressed as follows
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