LT3510FE datasheet(14/28 Pages) LINER

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LT3510

3510fc

may result in discontinuous mode operation, which is

okay, but further reduces maximum load current. For

details of maximum output current and discontinuous

mode operation, see Linear Technology Application Note

44. Finally, for duty cycles greater than 50% (VOUT/VIN

> 0.5), there is a minimum inductance required to avoid

subharmonic oscillations. See Application Note 19 for

more information.

Input Capacitor Selection

Bypass the inputs of the LT3510 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 capaci-

tor is required to reduce the resulting voltage ripple at the

LT3510 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 fre-

quency 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:

CIN RMS

OUT

()

•–

()

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 ~2A, RMS ripple current

will always be less than 1A.

The frequency, VIN to VOUT ratio, and maximum load cur-

rent requirement of the LT3510 along with the input supply

source impedance, determine the energy storage require-

ments of the input capacitor. Determine the worst-case

condition for input ripple current and then size the input

capacitor such that it reduces input voltage ripple to an

acceptable level. Typical values for input capacitors run

from 10μF at low frequencies to 2.2μF at higher frequencies.

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 capacitance, 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 by-

pass 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 example 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 require-

ments. Because the input capacitor is likely to see high

surge currents when the input source is applied, tantalum

capacitors should be surge rated. The manufacturer may

also recommend operation below the rated voltage of the

capacitor. Be sure to place the 1μF ceramic as close as

possible to the VIN and GND pins on the IC for optimal

noise immunity.

When the LT3510’s input supplies are operated at different

input voltages, an input capacitor sized for that channel

should be placed as close as possible to the respective

VIN pins.

A ﬁnal caution regarding the use of ceramic capacitors

at the input. A ceramic input capacitor can combine with

stray inductance to form a resonant tank circuit. If power

is applied quickly (for example by plugging the circuit

into a live power source) this tank can ring, doubling the

input voltage and damaging the LT3510. The solution is to

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