Datasheet

LM27341, LM27342, LM27341-Q1, LM27342-Q1
SNVS497E –NOVEMBER 2008–REVISED APRIL 2013 www.ti.com
First the maximum duty cycle is calculated.
D
MAX
= (V
OUT
+ V
D1
) / (V
IN
+ V
D1
- V
DS
)
= (3.3V + 0.5V) / (7V + 0.5V - 0.30V)
= 0.528 (14)
Using Figure 36 gives us a recommended ripple ratio = 0.4.
Now the minimum duty cycle is calculated.
D
MIN
= (V
OUT
+ V
D1
) / (V
IN
+ V
D1
- V
DS
)
= (3.3V + 0.5V) / (16V + 0.5V - 0.30V)
= 0.235 (15)
The inductance can now be calculated.
L = (1 - D
MIN
) x (V
OUT
+ V
D1
) / (I
OUT
x r x f
sw
)
= (1 - 0.235) x (3.3V + .5V) / (2A x 0.4 x 2 MHz)
= 1.817 µH (16)
This is close to the standard inductance value of 1.8 µH. This leads to a 1% deviation from the recommended
ripple ratio, which is now 0.4038.
Finally, we check that the peak current does not reach the minimum current limit of 2.5A.
I
LPK
= I
OUT
x (1 + r / 2)
= 2A x (1 + .4038 / 2 )
= 2.404A (17)
The peak current is less than 2.5A, so the DC load specification can be met with this ripple ratio. To design for
the LM27341 simply replace I
OUT
= 1.5A in the equations for I
LPK
and see that I
LPK
does not exceed the
LM27341's current limit of 2.0A (min).
INDUCTOR MATERIAL SELECTION
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. To prevent the inductor from saturating over the entire -40 °C to 125 °C range, pick an inductor with a
saturation current higher than the upper limit of I
CL
listed in the Electrical Characteristics table.
Ferrite core inductors are recommended to reduce AC loss and fringing magnetic flux. The drawback of ferrite
core inductors is their quick saturation characteristic. The current limit circuit has a propagation delay and so is
oftentimes not fast enough to stop a saturated inductor from going above the current limit. This has the potential
to damage the internal switch. To prevent a ferrite core inductor from getting into saturation, the inductor
saturation current rating should be higher than the switch current limit I
CL
. The LM27341/LM27342 is quite robust
in handling short pulses of current that are a few amps above the current limit. Saturation protection is provided
by a second current limit which is 30% higher than the cycle by cycle current limit. When the saturation protection
is triggered the part will turn off the output switch and attempt to soft-start. (When a compromise has to be made,
pick an inductor with a saturation current just above the lower limit of the I
CL
.) Be sure to validate the short-circuit
protection over the intended temperature range.
An inductor's saturation current is usually lower when hot. So consult the inductor vendor if the saturation current
rating is only specified at room temperature.
Soft saturation inductors such as the iron powder types can also be used. Such inductors do not saturate
suddenly and therefore are safer when there is a severe overload or even shorted output. Their physical sizes
are usually smaller than the Ferrite core inductors. The downside is their fringing flux and higher power
dissipation due to relatively high AC loss, especially at high frequencies.
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