I
RMS-IN
= I
O
x
D x
r
2
12
1-D +
L =
V
O
+ V
D
I
O
x r x f
S
x (1-D)
r =
'
i
L
l
O
LM2734
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SNVS288I – SEPTEMBER 2004 – REVISED APRIIL 2013
The inductor value determines the output ripple current. Lower inductor values decrease the size of the inductor,
but increase the output ripple current. An increase in the inductor value will decrease the output ripple current.
The ratio of ripple current (
Δ
i
L
) to output current (I
O
) is optimized when it is set between 0.3 and 0.4 at 1A. The
ratio r is defined as:
(15)
One must also ensure that the minimum current limit (1.2A) is not exceeded, so the peak current in the inductor
must be calculated. The peak current (I
LPK
) in the inductor is calculated by:
I
LPK
= I
O
+
Δ
I
L
/2
(16)
If r = 0.5 at an output of 1A, the peak current in the inductor will be 1.25A. The minimum specified current limit
over all operating conditions is 1.2A. One can either reduce r to 0.4 resulting in a 1.2A peak current, or make the
engineering judgement that 50mA over will be safe enough with a 1.7A typical current limit and 6 sigma limits.
When the designed maximum output current is reduced, the ratio r can be increased. At a current of 0.1A, r can
be made as high as 0.9. The ripple ratio can be increased at lighter loads because the net ripple is actually quite
low, and if r remains constant the inductor value can be made quite large. An equation empirically developed for
the maximum ripple ratio at any current below 2A is:
r = 0.387 x I
OUT
-0.3667
(17)
Note that this is just a guideline.
The LM2734 operates at frequencies allowing the use of ceramic output capacitors without compromising
transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple.
See the output capacitor section for more details on calculating output voltage ripple.
Now that the ripple current or ripple ratio is determined, the inductance is calculated by:
(18)
where f
s
is the switching frequency and I
O
is the output current. When selecting an inductor, make sure that it is
capable of supporting the peak output current without saturating. Inductor saturation will result in a sudden
reduction in inductance and prevent the regulator from operating correctly. Because of the speed of the internal
current limit, the peak current of the inductor need only be specified for the required maximum output current. For
example, if the designed maximum output current is 0.5A and the peak current is 0.7A, then the inductor should
be specified with a saturation current limit of >0.7A. There is no need to specify the saturation or peak current of
the inductor at the 1.7A typical switch current limit. The difference in inductor size is a factor of 5. Because of the
operating frequency of the LM2734, ferrite based inductors are preferred to minimize core losses. This presents
little restriction since the variety of ferrite based inductors is huge. Lastly, inductors with lower series resistance
(DCR) will provide better operating efficiency. For recommended inductors see Example Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that V
IN
does not drop excessively during switching transients. The
primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and ESL (Equivalent
Series Inductance). The recommended input capacitance is 10µF, although 4.7µF works well for input voltages
below 6V. The input voltage rating is specifically stated by the capacitor manufacturer. Make sure to check any
recommended deratings and also verify if there is any significant change in capacitance at the operating input
voltage and the operating temperature. The input capacitor maximum RMS input current rating (I
RMS-IN
) must be
greater than:
(19)
It can be shown from the above equation that maximum RMS capacitor current occurs when D = 0.5. Always
calculate the RMS at the point where the duty cycle, D, is closest to 0.5. The ESL of an input capacitor is usually
determined by the effective cross sectional area of the current path. A large leaded capacitor will have high ESL
and a 0805 ceramic chip capacitor will have very low ESL. At the operating frequencies of the LM2734, certain
capacitors may have an ESL so large that the resulting impedance (2
π
fL) will be higher than that required to
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