MBRM140 datasheet(12/20 Pages) LINER

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LT3474/LT3474-1

3474fd

APPLICATIONS INFORMATION

To maintain output regulation, this peak current must be

less than the LT3474’s switch current limit ILIM. For SW1,

ILIM is at least 1.6A (1.5A at 125°C) at low duty cycles and

decreases linearly to 1.15A (1.08A at 125°C) at DC = 0.8.

The maximum output current is a function of the chosen

inductor value:

OUT MAX

() =

LIM –

ΔIL

=1.6A • 1 – 0.35 •DC

() –

ΔIL

Choosing an inductor value so that the ripple current is

small will allow a maximum output current near the switch

current limit.

One approach to choosing the inductor is to start with the

simple rule given above, look at the available inductors,

and choose one to meet cost or space goals. Then use

these equations to check that the LT3474 will be able to

deliver the required output current. Note again that these

equations assume that the inductor current is continuous.

Discontinuous operation occurs when IOUT is less than

ΔIL/2.

Input Capacitor Selection

Bypass the input of the LT3474 circuit with a 2.2μF or

higher ceramic capacitor of X7R or X5R type. A lower

value or a less expensive Y5V type will work if there is

additional bypassing provided by bulk electrolytic capaci-

tors or if the input source impedance is low. The following

paragraphs describe the input capacitor considerations in

more detail.

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the LT3474 input and to force this switching current into

a tight local loop, minnimizing 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. The RMS input is:

INRMS

OUT

()

•

–

and is largest when VIN = 2VOUT (50% duty cycle). Con-

sidering that the maximum load current is 1A, RMS ripple

current will always be less than 0.5A

The high switching frequency of the LT3474 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 makes them the

preferred choice. The low ESR results in very low voltage

ripple. Ceramic capacitors can handle larger magnitudes

of ripple current than other capacitor types of the same

value. Use X5R and X7R types.

An alternative to a high value ceramic capacitor is a lower

value ceramic along with a larger electrolytic capaci-

tor. The electrolytic capacitor likely needs to be greater

than 10μF in order to meet the ESR and ripple current

requirements. The input capacitor is likely to see high

surge currents when the input source is applied. Tanta-

lum capacitors can fail due to an over-surge of current.

Only use tantalum capacitors with the appropriate surge

current rating. The manufacturer may also recommend

operation below the rated voltage of the capacitor.

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