Datasheet

LM26400Y
SNVS457C –FEBRUARY 2007–REVISED APRIL 2013 www.ti.com
Figure 36. Capacitance vs. Applied DC Voltage
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 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)
-1
= 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 capacitor 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 capacitors, 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
capacitors 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 electrolytic 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 temperature in the real thermal environment to
make sure the capacitors 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, calculation of the input ripple RMS current is a little
tedious. Use the following equation.
(21)
I
1
is Channel 1's maximum output current. I
2
is Channel 2's maximum output current. d1 is the non-overlapping
portion of Channel 1's duty cycle D
1
. d2 is the non-overlapping portion of Channel 2's duty cycle D
2
. d3 is the
overlapping portion of the two duty cycles. I
av
is the average input current. I
av
= I
1
·D
1
+ I
2
·D
2
. To quickly determine
the values of d1, d2 and d3, refer to the decision tree in Figure 37. To determine the duty cycle of each channel,
use D = V
OUT
/V
IN
for a quick result or use the following equation for a more accurate result.
(22)
R
DC
is the winding resistance of the inductor. R
DS
is the ON resistance of the MOSFET switch.
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