lm27342sdx National Semiconductor Corporation, lm27342sdx Datasheet - Page 13

• Manufacturer Part Number: lm27342sdx
• Description: 2 Mhz 1.5a/2a Wide Input Range Step-down Dc-dc Regulator With Frequency Synchronization
• Manufacturer: National Semiconductor Corporation

a guideline. Please see Application note AN-1197 for further
considerations.
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (V
The application's lowest input voltage should be used to cal-
culate the ripple ratio. The catch diode forward voltage drop
(V
must be included to calculate a more accurate duty cycle.
Calculate D by using the following formula:
V
The diode forward drop (V
depending on the quality of the diode. The lower V
higher the operating efficiency of the converter.
Now that the ripple current or ripple ratio is determined, the
required inductance is calculated by:
where D
input voltage, f
maximum output current of 2A. Using I
the inductor's physical size.
INDUCTOR CALCULATION EXAMPLE
Operating conditions for the LM27342 are:
First the maximum duty cycle is calculated.
V
f
FIGURE 6. Recommended Ripple Ratio Vs. Duty Cycle
SW
DS
IN
D1
) and the voltage drop across the internal NFET (V
= 7 - 16V
can be approximated by:
= 2 MHz
MIN
is the duty cycle calculated with the maximum
sw
is the switching frequency, and I
V
V
V
DS
OUT
D1
= I
OUT
= 0.5V
= 3.3V
OUT
D1
) to input voltage (V
) can range from 0.3V to 0.5V
x R
DS(ON)
OUT
I
OUT
= 2A will minimize
= 2A
IN
30005627
):
OUT
D1
is, the
is the
DS
)
13
Using Figure 6 gives us a recommended ripple ratio = 0.4.
Now the minimum duty cycle is calculated.
The inductance can now be calculated.
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.
The peak current is less than 2.5A, so the DC load specifica-
tion can be met with this ripple ratio. To design for the
LM27341 simply replace I
I
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. Induc-
tor 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
ble.
Ferrite core inductors are recommended to reduce AC loss
and fringing magnetic flux. The drawback of ferrite core in-
ductors 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 sat-
uration, the inductor saturation current rating should be higher
than the switch current limit I
quite robust in handling short pulses of current that are a few
amps above the current limit. Saturation protection is provid-
ed 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 in-
ductor with a saturation current just above the lower limit of
the I
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
D
D
L
I
LPK
LPK
MAX
MIN
and see that I
CL
= (1 - D
= (1 - 0.235) x (3.3V + .5V) / (2A x 0.4 x 2 MHz)
= 1.817 µH
= I
= 2A x (1 + .4038 / 2 )
= 2.404A
.) Be sure to validate the short-circuit protection over
OUT
= (V
= (3.3V + 0.5V) / (7V + 0.5V - 0.30V)
= 0.528
= (V
= (3.3V + 0.5V) / (16V + 0.5V - 0.30V)
= 0.235
x (1 + r / 2)
MIN
OUT
OUT
) x (V
LPK
CL
+ V
+ V
listed in the Electrical Characteristics ta-
does not exceed the LM27341's current
D1
D1
OUT
) / (V
) / (V
+ V
OUT
IN
IN
D1
CL
+ V
+ V
) / (I
= 1.5A in the equations for
. The LM27341/LM27342 is
D1
D1
OUT
- V
- V
DS
DS
x r x f
)
)
sw
)
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