lm8801tmx National Semiconductor Corporation, lm8801tmx Datasheet - Page 11

• Manufacturer Part Number: lm8801tmx
• Description: High Precision 6mhz, 600 Ma Synchronous Step-down Dc-dc Converter For Mobile Applications
• Manufacturer: National Semiconductor Corporation

Application Information
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor should not saturate, and the inductor current
ripple is small enough to achieve the desired output voltage
ripple. Different manufacturers follow different saturation cur-
rent rating specifications, so attention must be given to de-
tails. Saturation current ratings are typically specified at 25°C
so ratings at maximum ambient temperature of application
should be requested from manufacturer.
Minimum value of inductance to guarantee good perfor-
mance is 0.3 µH at (I
temp range.
Shielded inductors radiate less noise and should be pre-
ferred. There are two methods to choose the inductor satu-
ration current rating.
Method 1:
The saturation current should be greater than the sum of the
maximum load current and the worst case average to peak
inductor current. This can be written as:
•
•
•
•
•
OUTPUT CAPACITOR SELECTION
Use a 4.7 μF, 6.3V ceramic capacitor, X7R or X5R types; do
not use Y5V. DC bias characteristics of ceramic capacitors
must be considered when selecting case sizes 0402 and
0603. DC bias characteristics vary from manufacturer to man-
ufacturer, and DC bias curves should be requested from them
as part of the capacitor selection process. Minimum output
capacitance to guarantee good performance is 2.2 µF (for
4.7 µF capacitor) at 1.8V DC bias including tolerances and
over ambient temp range.
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 ca-
pacitance and sufficiently low ESR to perform these functions.
The output voltage ripple is caused by the charging and dis-
charging of the output capacitor and also due to its R
can be calculated as:
Voltage peak-to-peak ripple due to capacitance =
I
I
V
L: minimum inductor value including worst case tolerances
(30% drop can be considered for method 1)
f: minimum switching frequency (5.4 MHz)
RIPPLE
OUTMAX
IN
: maximum input voltage in application
: average to peak inductor current
: maximum load current (600 mA)
LQM21PNR54MG0D
MIPSZ2012D0R5
LIM
Model
typ.) bias current over the ambient
TABLE 1. Suggested Inductors and Suppliers
ESR
and
11
Vendor
Murata
FDK
•
Method 2:
A more conservative and recommended approach is to
choose an inductor that can handle the maximum current limit
of 1300 mA.
The inductor's resistance should be less than around 0.1Ω for
good efficiency. Table 1 lists suggested inductors and sup-
pliers.
INPUT CAPACITOR SELECTION
A ceramic input capacitor of 2.2 µF, 6.3V is sufficient for most
applications. Place the input capacitor as close as possible to
the V
ing may be used to improve input voltage filtering. Use X7R
or X5R types; do not use Y5V. DC bias characteristics of ce-
ramic capacitors must be considered when selecting case
sizes like 0402 and 0603.
The input filter capacitor supplies current to the top switch of
the LM8801 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. Se-
lect an input filter capacitor with sufficient ripple current rating.
The input current ripple can be calculated as:
The worst case is when V
Voltage peak-to-peak ripple due to ESR =
V
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 =
Note that the output voltage ripple is dependent on the current
ripple and the equivalent series resistance of the output ca-
pacitor (R
temperature dependent); make sure the value used for cal-
culations is at the switching frequency of the part.
Table 2 lists suggested capacitors and suppliers.
PP-ESR
V
OUT
IN
pin of the device. A larger value or higher voltage rat-
: output voltage
= (2 * I
ESR
). The R
RIPPLE
) * R
ESR
Dimensions LxWxH (mm)
ESR
is frequency dependent (as well as
IN
= 2 x V
2.0 x 1.2 x 1.0
2.0 x 1.2 x 0.9
OUT
.
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