LM26400YMH NSC [National Semiconductor], LM26400YMH Datasheet - Page 16

• Manufacturer Part Number: LM26400YMH
• Description: Dual 2A, 500kHz Wide Input Range Buck Regulator
• Manufacturer: NSC [National Semiconductor]

www.national.com
Choose a 5µH or so ferrite core inductor that has a saturation
current around 3A at room temperature. For example,
Sumida's CDRH6D26NP-5R0NC.
If the maximum load current is significantly lower than 2A, pick
an inductor with the same saturation rating as a 2A design but
with a lowered DC current rating. That should result in a
smaller inductor. There are not many choices, though. An-
other possibility is to use a soft saturation type inductor,
whose size will be dominated by the DC current rating.
OUTPUT CAPACITOR SELECTION
Output capacitors in a buck regulator handles the AC current
from the inductor and so have little ripple RMS current and
their power dissipation is not a concern. The concern usually
revolves around loop stability and capacitance retention.
The LM26400Y's internal loop compensation was designed
around ceramic output capacitors. From a stability point of
view, the lower the output voltage, the more capacitance is
needed.
Below is a quick summary of temperature characteristics of
some commonly used ceramic capacitors. So an X7R ceram-
ic capacitor means its capacitance can vary ±15% over the
temperature range of -55°C to +125°C.
Besides the variation of capacitance over temperature, the
actual capacitance of ceramic capacitors also vary, some-
times significantly, with applied DC voltage. Figure 7 illus-
trates such a characteristic of several ceramic capacitors of
various physical sizes from Murata. Unless the DC voltage
across the capacitor is going to be small relative to its rated
value, going to too small a physical size will have the penalty
of losing significant capacitance during circuit operation.
The amount of output capacitance directly contributes to the
output voltage ripple magnitude. A quick way to estimate the
output voltage ripple is to multiply the inductor peak-to-peak
Low
Temperature
X: -55°C
Y: -30°C
Z: +10°C
Capacitance Variation Over Temperature (Class II
FIGURE 7. Capacitance vs. Applied DC Voltage
Dielectric Ceramic Capacitors)
High
Temperature
5: +85°C
6: +105°C
7: +125°C
8: +150°C
Capacitance Change
Range
R: ±15%
S: ±22%
U: +22%, -56%
V: +22%, -82%
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16
ripple current by the impedance of the output capacitors. For
example, if the inductor ripple current is 0.6A peak-to-peak,
and the output capacitance is 44µF, then the output voltage
ripple should be close to 0.6A x (6.28 x 500kHz x 44µF)
4.3mV. Sometimes when a large ceramic capacitor is used,
the switching frequency may be higher than the capacitor's
self resonance frequency. In that case, find out the true
impedance at the switching frequency and then multiply that
value by the ripple current to get the ripple voltage.
The amount of output capacitance also impacts the stability
of the feedback loop. Refer to the LOOP STABILITY section
for guidelines.
INPUT CAPACITOR SELECTION
The input capacitors provide the AC current needed by the
nearby power switch so that current provided by the upstream
power supply does not carry a lot of AC content, generating
less EMI. To the buck regulator in question, the input capac-
itor also prevents the drain voltage of the FET switch from
dipping when the FET is turned on, therefore providing a
healthy line rail for the LM26400Y to work with. Since typically
most of the AC current is provided by the local input capaci-
tors, the power loss in those capacitors can be a concern. In
the case of the LM26400Y regulator, since the two channels
operate 180° out of phase, the AC stress in the input capac-
itors is less than if they operated in phase. The measure for
the AC stress is called input ripple RMS current. It is strongly
recommended that at least one 4.7µF ceramic capacitor be
placed next to the PVIN pins. Bulk capacitors such as elec-
trolytic capacitors or OSCON capacitors can be added to help
stabilize the local line voltage, especially during large load
transient events. As for the ceramic capacitors, use X7R , X6S
or X5R types. They maintain most of their capacitance over
a wide temperature range. Try to avoid sizes smaller than
0805. Otherwise significant drop in capacitance may be
caused by the DC bias voltage. See OUTPUT CAPACITOR
SELECTION section for more information. The DC voltage
rating of the ceramic capacitor should be higher than the
highest input voltage.
Capacitor temperature is a major concern in board designs.
While using a 4.7µF or higher MLCC as the input capacitor is
a good starting point, it is a good idea to check the tempera-
ture in the real thermal environment to make sure the capac-
itors are not over heated. Capacitor vendors may provide
curves of ripple RMS current vs. temperature rise, based on
a designated thermal impedance. In reality, the thermal
impedance may be very different. So it is always a good idea
to check the capacitor temperature on the board.
Since the duty cycles of the two channels may overlap, cal-
culation of the input ripple RMS current is a little tedious. Use
the following equation.
I
maximum output current. d1 is the non-overlapping portion of
Channel 1's duty cycle D
of Channel 2's duty cycle D
the two duty cycles. I
I
d3, refer to the decision tree in Figure 8. To determine the
duty cycle of each channel, use D = V
or use the following equation for a more accurate result.
1
1
·D
is Channel 1's maximum output current. I
1
+ I
2
·D
2
. To quickly determine the values of d1, d2 and
av
1
is the average input current. I
. d2 is the non-overlapping portion
2
. d3 is the overlapping portion of
OUT
/V
IN
2
for a quick result
is Channel 2's
-1
av
=
=