LM3677_0810 Datasheet, PDF (16/24 Pages) National Semiconductor (TI) – 3MHz, 600mA Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits

up times with a 10 µF output capacitor and 300 mA load is
300 µs and with 1 mA load is 200 µs.
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 0.7 µH at I_LIM (typ.) DC
current over the ambient temperature range. Shielded in-
ductors 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
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_IN pin of the device. A larger value may be used for im-
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 2.2 µF at 3V DC bias; 1.5 µF 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:
• I_RIPPLE: average to peak inductor current
• I_OUTMAX: maximum load current (600 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 (2.5 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 1375 mA.
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