Datasheet

LM26400Y
www.ti.com SNVS457C –FEBRUARY 2007–REVISED APRIL 2013
Ferrite core inductors are recommended for less AC loss and less fringing magnetic flux. The drawback of ferrite
core inductors is their quick saturation characteristic. Once the inductor gets saturated, its current can spike up
very quickly if the switch is not turned off immediately. The current limit circuit has a propagation delay and so is
oftentimes not fast enough to stop the saturated inductor from going above the current limit. This has the
potential to damage the internal switch. So 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 LM26400Y is quite robust
in handling short pulses of current that is a few amps above the current limit. 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.
To prevent the inductor from saturating over the entire -40°C to 125°C range, pick one with a saturation current
higher than the upper limit of I
CL
in the Electrical Characteristics table.
Inductor 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.
Example:
V
OUT
= 1.2V; V
IN
= 9V to 14V; I
OUT
= 2A max; Peak-to-peak Ripple Current ΔI = 0.6A.
(20)
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. Another 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 ceramic capacitor means its capacitance can vary ±15% over the temperature range of -55°C to +125°C.
Table 1. Capacitance Variation Over Temperature (Class II Dielectric Ceramic Capacitors)
Low Temperature High Temperature Capacitance Change Range
X: -55°C 5: +85°C R: ±15%
Y: -30°C 6: +105°C S: ±22%
Z: +10°C 7: +125°C U: +22%, -56%
8: +150°C V: +22%, -82%
Besides the variation of capacitance over temperature, the actual capacitance of ceramic capacitors also vary,
sometimes significantly, with applied DC voltage. Figure 36 illustrates 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.
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