LM27341_15 datasheet(16/44 Pages) TI

16 / 44 page

LM27341, LM27342, LM27341-Q1, LM27342-Q1

SNVS497E – NOVEMBER 2008 – REVISED APRIL 2013

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First the maximum duty cycle is calculated.

DMAX = (VOUT + VD1) / (VIN + VD1 - VDS)

= (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.

DMIN = (VOUT + VD1) / (VIN + VD1 - VDS)

= (3.3V + 0.5V) / (16V + 0.5V - 0.30V)

= 0.235

(15)

The inductance can now be calculated.

L = (1 - DMIN) x (VOUT + VD1) / (IOUT x r x fsw)

= (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.

ILPK = IOUT 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 IOUT = 1.5A in the equations for ILPK and see that ILPK 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 ICL 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 ICL. 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 ICL.) 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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