Datasheet

V
PP-C
=
f*8*C
I
PP
I
RMS
= I
OUTMAX
The worst case IRMS is:
I
RMS
=
V
OUT
V
IN
1 -
V
OUT
V
IN
¸
¸
¹
·
¨
¨
©
§
I
OUTMAX
2
(duty cycle = 50%)
*
*
L >= (
1
f
)
V
IN
- V
OUT
) * ( ) * (
I
PP
V
OUT
V
IN
LM3370
SNVS406N –NOVEMBER 2005–REVISED MAY 2013 www.ti.com
(8)
A 2.2 µH inductor with a saturation current rating of at least 1400 mA is recommended for most applications. The
inductor’s resistance should be less than around 0.2Ω for good efficiency. Table 4 lists suggested inductors and
suppliers.
For low-cost applications, an unshielded bobbin inductor is suggested. 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 toroidal inductor, in the event that noise
from low-cost bobbin models is unacceptable.
Below are some suggested inductor manufacturers that include but are not limited to:
Table 4. Suggested Inductors and Suppliers
Model Vendor Dimensions (mm) I
SAT
DO3314-222 Coilcraft 3.3 x 3.3 x 1.4 1.6A
LPO3310-222 3.3 x 3.3 x 1.0 1.1A
SD3114-2R2 Cooper 3.1 x 3.1 x 1.4 1.48A
NR3010T2R2M Taiyo Yuden 3.0 x 3.0 x 1.0 1.1A
NR3015T2R2M 3.0 x 3.0 x 1.5 1.48A
VLF3010AT- 2R2M1R0 TDK 2.6 x 2.8 x 1.0 1.0A
Input Capacitor Selection
A ceramic input capacitor of 4.7 μF, 6.3V is sufficient for most applications. A larger value may be used for
improved input voltage filtering. The input filter capacitor supplies current to the PFET switch of the LM3370 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
an input filter capacitor with a surge current rating sufficient for the power-up surge from the input power source.
The power-up surge current is approximately the capacitor’s value (µF) times the voltage rise rate (V/µs).
The input current ripple can be calculated as:
(9)
Output Capacitor Selection
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 ripple voltage can be
calculated as:
Voltage peak-to-peak ripple due to capacitance =
(10)
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