Datasheet

I
RMS OUTMAX
*
* (1 -
)
V
OUT
V
IN
V
OUT
V
IN
= I
The worst case IRMS is:
=
2
IRMS
IRMS
(duty cycle = 50%)
) * (
1
f
)) * (L >= (
V
IN
- V
OUT
I
PP
V
OUT
V
IN
V
OUT
=
V
IN
-V
OUT
2 L
I
LOAD
+
(
)
V
OUT
V
IN
(
)
(
1
f
)
*
I
MAX
=
I
RIPPLE
2
I
LOAD
+
LM3670
SNVS250E –NOVEMBER 2004–REVISED FEBRUARY 2013 www.ti.com
A pole can also be used at higher output voltages. For example, in Table 3, there is an entry for 1.24V with both
a pole and zero at approximately 10kHz for noise rejection.
Inductor Selection
There are two main considerations when choosing an inductor; the inductor current should not saturate, and the
inductor current ripple is small enough to achieve the desired output voltage ripple.
There are two methods to choose the inductor current rating.
Method 1:
The total current is the sum of the load and the inductor ripple current. This can be written as
(9)
where
• I
LOAD
load current
• V
IN
input voltage
• L inductor
• f switching frequency
• I
RIPPLE
peak-to-peak (10)
Method 2:
A more conservative approach is to choose an inductor that can handle the current limit of 700 mA.
Given a peak-to-peak current ripple (I
PP
) the inductor needs to be at least
(11)
A 10 µH inductor with a saturation current rating of at least 800 mA is recommended for most applications. The
inductor’s resistance should be less than around 0.3Ω for good efficiency. Table 1 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.
Input Capacitor Selection
A ceramic input capacitor of 4.7 µF 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 LM3670 in the first half
of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capcitor’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:
(12)
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