Datasheet

I
O-RMS
= 1.13 x I
L
x
D x (1 - D)
I
D
V
O
Âv
O
LM5022
www.ti.com SNVS480G –JANUARY 2007–REVISED DECEMBER 2013
Figure 17. ΔV
O
Using Low ESR Capacitors
For this example the small size and high temperature rating of ceramic capacitors make them a good choice.
The output ripple voltage waveform of Figure 17 is assumed, and the capacitance will be selected first. The
desired ΔV
O
is ±2% of 40V, or 0.8V
P-P
. Beginning with the calculation for ΔV
O2
, the required minimum
capacitance is:
C
O-MIN
= (I
O
/ ΔV
O
) x (D
MAX
/ f
SW
) C
O-MIN
= (0.5 / 0.8) x (0.77 / 5 x 10
5
) = 0.96 µF (24)
The next higher standard 20% capacitor value is 1.0 µF, however to provide margin for component tolerance and
load transients two capacitors rated 4.7 µF each will be used. Ceramic capacitors rated 4.7 µF ±20% are
available from many manufacturers. The minimum quality dielectric that is suitable for switching power supply
output capacitors is X5R, while X7R (or better) is preferred. Careful attention must be paid to the DC voltage
rating and case size, as ceramic capacitors can lose 60% or more of their rated capacitance at the maximum DC
voltage. This is the reason that ceramic capacitors are often de-rated to 50% of their capacitance at their working
voltage. The output capacitors for this example will have a 100V rating in a 2220 case size.
The typical ESR of the selected capacitors is 3 mΩ each, and in parallel is approximately 1.5 mΩ. The worst-
case value for ΔV
O1
occurs during the peak current at minimum input voltage:
ΔV
O1
= 2.5 x 0.0015 = 4 mV (25)
The worst-case capacitor charging ripple occurs at maximum duty cycle:
ΔV
O2
= (0.5 / 9.4 x 10
-6
) x (0.77 / 5 x 10
5
) = 82 mV (26)
Finally, the worst-case value for ΔV
O3
occurs when inductor ripple current is highest, at maximum input voltage:
ΔV
O3
= 0.58 x 0.0015 = 1 mV (negligible) (27)
The output voltage ripple can be estimated by summing the three terms:
ΔV
O
= 4 mV + 82 mV - 1 mV = 85 mV (28)
The RMS current through the output capacitor(s) can be estimated using the following, worst-case equation:
(29)
The highest RMS current occurs at minimum input voltage. For this example the maximum output capacitor RMS
current is:
I
O-RMS(MAX)
= 1.13 x 2.3 x (0.78 x 0.22)
0.5
= 1.08A
RMS
(30)
These 2220 case size devices are capable of sustaining RMS currents of over 3A each, making them more than
adequate for this application.
VCC DECOUPLING CAPACITOR
The VCC pin should be decoupled with a ceramic capacitor placed as close as possible to the VCC and GND
pins of the LM5022. The decoupling capacitor should have a minimum X5R or X7R type dielectric to ensure that
the capacitance remains stable over voltage and temperature, and be rated to a minimum of 470 nF. One good
choice is a 1.0 µF device with X7R dielectric and 1206 case size rated to 25V.
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