Datasheet

C
MIN
=
2 x 1P x 40 x 0.5
9
2
x 0.1
= 4.9 PF
C
MIN
=
2 x L
S
x V
O
x I
O
V
IN
2
x R
S
ESR
MIN
=
(1-D)x'v
IN
2 x I
STEP
LM5022
SNVS480G –JANUARY 2007–REVISED DECEMBER 2013 www.ti.com
INPUT CAPACITOR
The input capacitors to a boost regulator control the input voltage ripple, ΔV
IN
, hold up the input voltage during
load transients, and prevent impedance mismatch (also called power supply interaction) between the LM5022
and the inductance of the input leads. Selection of input capacitors is based on their capacitance, ESR, and RMS
current rating. The minimum value of ESR can be selected based on the maximum output current transient,
I
STEP
, using the following expression:
(31)
For this example the maximum load step is equal to the load current, or 0.5A. The maximum permissable ΔV
IN
during load transients is 4%
P-P
. ΔV
IN
and duty cycle are taken at minimum input voltage to give the worst-case
value:
ESR
MIN
= [(1 – 0.77) x 0.36] / (2 x 0.5) = 83 mΩ (32)
The minimum input capacitance can be selected based on ΔV
IN
, based on the drop in V
IN
during a load transient,
or based on prevention of power supply interaction. In general, the requirement for greatest capacitance comes
from the power supply interaction. The inductance and resistance of the input source must be estimated, and if
this information is not available, they can be assumed to be 1 µH and 0.1Ω, respectively. Minimum capacitance
is then estimated as:
(33)
As with ESR, the worst-case, highest minimum capacitance calculation comes at the minimum input voltage.
Using the default estimates for L
S
and R
S
, minimum capacitance is:
(34)
The next highest standard 20% capacitor value is 6.8 µF, but because the actual input source impedance and
resistance are not known, two 4.7 µF capacitors will be used. In general, doubling the calculated value of input
capacitance provides a good safety margin. The final calculation is for the RMS current. For boost converters
operating in CCM this can be estimated as:
I
RMS
= 0.29 x Δi
L(MAX)
(35)
From the inductor section, maximum inductor ripple current is 0.58A, hence the input capacitor(s) must be rated
to handle 0.29 x 0.58 = 170 mA
RMS
.
The input capacitors can be ceramic, tantalum, aluminum, or almost any type, however the low capacitance
requirement makes ceramic capacitors particularly attractive. As with the output capacitors, the minimum quality
dielectric used should X5R, with X7R or better preferred. The voltage rating for input capacitors need not be as
conservative as the output capacitors, as the need for capacitance decreases as input voltage increases. For this
example, the capacitor selected will be 4.7 µF ±20%, rated to 50V, in the 1812 case size. The RMS current
rating of these capacitors is over 2A each, more than enough for this application.
CURRENT SENSE FILTER
Parasitic circuit capacitance, inductance and gate drive current create a spike in the current sense voltage at the
point where Q1 turns on. In order to prevent this spike from terminating the on-time prematurely, every circuit
should have a low-pass filter that consists of C
CS
and R
S1
, shown in Figure 12. The time constant of this filter
should be long enough to reduce the parasitic spike without significantly affecting the shape of the actual current
sense voltage. The recommended range for R
S1
is between 10Ω and 500Ω, and the recommended range for C
CS
is between 100 pF and 2.2 nF. For this example, the values of R
S1
and C
CS
will be 100Ω and 1 nF, respectively.
18 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LM5022