LT1940/LT1940L
U
W U U
APPLICATIO S I FOR ATIO
Table 1. Inductors.
Part Number
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
switchingfrequencytodothiseffectively, anditmusthave
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 (VOUT • IOUT). This is given by:
Value
(µH)
I
DCR
(Ω)
Height
(mm)
SAT
(A) DC
Sumida
CR43-1R4
1.4
2.2
3.3
4.7
1.5
2.2
3.3
3.3
4.7
5.3
4.1
2.52
1.75
1.44
1.15
1.55
1.20
1.10
1.57
1.32
1.9
0.056
0.071
0.086
0.109
0.040
0.050
0.063
0.049
0.072
0.028
0.042
3.5
3.5
3.5
3.5
1.8
1.8
1.8
3.0
3.0
3.0
2.0
CR43-2R2
CR43-3R3
CR43-4R7
CDRH3D16-1R5
CDRH3D16-2R2
CDRH3D16-3R3
CDRH4D28-3R3
CDRH4D28-4R7
CDRH5D28-5R3
CDRH5D18-4R1
Coilcraft
C
INRMS = IOUT √[VOUT • (VIN – VOUT)]/VIN < IOUT/2
and is largest when VIN = 2VOUT (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.
1.95
DO1606T-152
DO1606T-222
DO1606T-332
DO1606T-472
DO1608C-152
DO1608C-222
DO1608C-332
DO1608C-472
1812PS-222M
1008PS-182M
Murata
1.5
2.2
3.3
4.7
1.5
2.2
3.3
4.7
2.2
1.8
2.10
1.70
1.30
1.10
2.60
2.30
2.00
1.50
1.7
0.060
0.070
0.100
0.120
0.050
0.070
0.080
0.090
0.070
0.090
2.0
2.0
2.0
2.0
2.9
2.9
2.9
2.9
3.81
2.74
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
usedinthisapplicationattheirratedvoltage. X5RandX7R
typesarestableovertemperatureandappliedvoltage, 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
maybecomefairlylarge,andtheripplecurrentmayendup
flowing from your input supply or from other bypass
capacitors in your system, as opposed to being fully
sourced from the local input capacitor.
2.1
LQH32CN1R0M11L
LQH32CN2R2M11L
LQH43CN1R5M01L
LQH43CN2R2M01L
LQH43CN3R3M01L
1.0
2.2
1.5
2.2
3.3
1.00
0.79
1.00
0.90
0.80
0.078
0.126
0.090
0.110
0.130
2.2
2.2
2.8
2.8
2.8
Input Capacitor Selection
Bypass the input of the LT1940 circuit with a 4.7µF or
higherceramiccapacitorofX7RorX5Rtype.Alowervalue
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.
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
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
ripple current requirements. Because the input capacitor
1940fa
9
Linear [ Linear ]
