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LT1940

1940i

Input Capacitor Selection

Bypass the input of the LT1940 circuit with a 4.7

µF 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 (VOUT • IOUT). This is given by:

CINRMS = 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.

The high frequency of the LT1940 reduces the energy

storage requirements of the input capacitor, so that the

capacitance required is less than 10

µF. 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 1

µF ceramic capacitor in parallel with a low-ESR

tantalum capacitor. For the electrolytic capacitor, a value

larger than 10

µF will be required to meet the ESR and

ripple current requirements. Because the input capacitor

APPLICATIO S I FOR ATIO

Table 1. Inductors.

Part Number

Value

ISAT

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

LQH3C1R0M24

1.0

1.00

0.078

2.2

LQH3C2R2M24

2.2

0.79

0.126

2.2

LQH4C1R5M04

1.5

1.00

0.090

2.8

LQH4C2R2M04

2.2

0.90

0.110

2.8

LQH4C3R3M04

3.3

0.80

0.130

2.8

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