LM3677TLX-ADJ National Semiconductor Corporation, LM3677TLX-ADJ Datasheet - Page 15

• Manufacturer Part Number: LM3677TLX-ADJ
• Description: 3mhz, 600ma Miniature Step-down Dc-dc Converter For Ultra Low Voltage Circuits
• Manufacturer: National Semiconductor Corporation

up times with a 10µF output capacitor and 300mA load is 300
µs and with 1mA load is 200µs.
Application Information
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor should not saturate, and the inductor current
ripple should be small enough to achieve the desired output
voltage ripple. Different saturation current rating specifica-
tions are followed by different manufacturers so attention
must be given to details. Saturation current ratings are typi-
cally specified at 25°C. However, ratings at the maximum
ambient temperature of application should be requested form
the manufacturer. The minimum value of inductance to
guarantee good performance is 0.7µH at I
rent over the ambient temperature range. Shielded induc-
tors radiate less noise and should be preferred.
There are two methods to choose the inductor saturation cur-
rent rating.
Method 1:
The saturation current is greater than the sum of the maxi-
mum load current and the worst case average to peak induc-
tor current. This can be written as
•
•
•
•
•
•
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 1375mA.
I
I
V
L : min inductor value including worst case tolerances
(30% drop can be considered for method 1)
f : minimum switching frequency (2.5MHz)
V
RIPPLE
OUTMAX
IN
OUT
: maximum input voltage in application
: output voltage
: average to peak inductor current
: maximum load current (600mA)
LIM
(typ) dc cur-
15
A 1.0 µH inductor with a saturation current rating of at least
1375 mA is recommended for most applications. The
inductor’s resistance should be less than 0.15Ω for good ef-
ficiency. Table 1 lists suggested inductors and suppliers. For
low-cost applications, an unshielded bobbin inductor could be
considered. For noise critical applications, a toroidal or shield-
ed-bobbin inductor should be used. A good practice is to lay
out the board with overlapping footprints of both types for de-
sign 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 4.7 µF, 6.3V is sufficient for most
applications. Place the input capacitor as close as possible to
the V
proved 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 0603 and 0805.
The minimum input capacitance to guarantee good per-
formance is 2.2µF at 3V dc bias; 1.5µF at 5V dc bias
including tolerances and over ambient temperature
range. The input filter capacitor supplies current to the PFET
switch of the LM3677 in the first half of each cycle and re-
duces 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 cur-
rent. Select a capacitor with sufficient ripple current rating.
The input current ripple can be calculated as:
IN
pin of the device. A larger value may be used for im-
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