LT1940/LT1940L
9
1940fa
Input Capacitor Selection
Bypass the input of the LT1940 circuit with a 4.7µF or
higher ceramic capacitor of X7R or X5R type. A lower value
or a less expensive Y5V type can be used if there is
additional bypassing provided by bulk electrolytic or
tantalum capacitors. The following paragraphs describe
the input capacitor considerations in more detail.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT1940 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
The input capacitor must have low impedance at the
switching frequency to do this effectively, and it must have
an adequate ripple current rating. With two switchers
operating at the same frequency but with different phases
and duty cycles, calculating the input capacitor RMS
current is not simple. However, a conservative value is the
RMS input current for the channel that is delivering most
power (V
OUT
• I
OUT
). This is given by:
C
INRMS
= I
OUT
√[V
OUT
• (V
IN
– V
OUT
)]/V
IN
< I
OUT
/2
and is largest when V
IN
= 2V
OUT
(50% duty cycle). As the
second, lower power channel draws input current, the
input capacitor’s RMS current actually decreases as the
out-of-phase current cancels the current drawn by the
higher power channel. Considering that the maximum
load current from a single channel is ~1.4A, RMS ripple
current will always be less than 0.7A.
The high frequency of the LT1940 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is less than 10µF. The combination
of small size and low impedance (low equivalent series
resistance or ESR) of ceramic capacitors make them the
preferred choice. The low ESR results in very low voltage
ripple and the capacitors can handle plenty of ripple
current. They are also comparatively robust and can be
used in this application at their rated voltage. X5R and X7R
types are stable over temperature and applied voltage, and
give dependable service. Other types (Y5V and Z5U) have
very large temperature and voltage coefficients of capaci-
tance, so they may have only a small fraction of their
nominal capacitance in your application. While they will
still handle the RMS ripple current, the input voltage ripple
may become fairly large, and the ripple current may end up
flowing from your input supply or from other bypass
capacitors in your system, as opposed to being fully
sourced from the local input capacitor.
An alternative to a high value ceramic capacitor is a lower
value along with a larger electrolytic capacitor, for ex-
ample a 1µF ceramic capacitor in parallel with a low ESR
tantalum capacitor. For the electrolytic capacitor, a value
larger than 10µF will be required to meet the ESR and
ripple current requirements. Because the input capacitor
APPLICATIO S I FOR ATIO
WUU
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Table 1. Inductors.
Part Number Value I
SAT
DCR Height
(µH) (A) DC (Ω) (mm)
Sumida
CR43-1R4 1.4 2.52 0.056 3.5
CR43-2R2 2.2 1.75 0.071 3.5
CR43-3R3 3.3 1.44 0.086 3.5
CR43-4R7 4.7 1.15 0.109 3.5
CDRH3D16-1R5 1.5 1.55 0.040 1.8
CDRH3D16-2R2 2.2 1.20 0.050 1.8
CDRH3D16-3R3 3.3 1.10 0.063 1.8
CDRH4D28-3R3 3.3 1.57 0.049 3.0
CDRH4D28-4R7 4.7 1.32 0.072 3.0
CDRH5D28-5R3 5.3 1.9 0.028 3.0
CDRH5D18-4R1 4.1 1.95 0.042 2.0
Coilcraft
DO1606T-152 1.5 2.10 0.060 2.0
DO1606T-222 2.2 1.70 0.070 2.0
DO1606T-332 3.3 1.30 0.100 2.0
DO1606T-472 4.7 1.10 0.120 2.0
DO1608C-152 1.5 2.60 0.050 2.9
DO1608C-222 2.2 2.30 0.070 2.9
DO1608C-332 3.3 2.00 0.080 2.9
DO1608C-472 4.7 1.50 0.090 2.9
1812PS-222M 2.2 1.7 0.070 3.81
1008PS-182M 1.8 2.1 0.090 2.74
Murata
LQH32CN1R0M11L 1.0 1.00 0.078 2.2
LQH32CN2R2M11L 2.2 0.79 0.126 2.2
LQH43CN1R5M01L 1.5 1.00 0.090 2.8
LQH43CN2R2M01L 2.2 0.90 0.110 2.8
LQH43CN3R3M01L 3.3 0.80 0.130 2.8
