3570 datasheet(13/20 Pages) LINER

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LT3570

3570fa

into a tight local loop, minimizing EMI. The input capaci-

tor must have low impedance at the switching frequency

to do this effectively and it must have an adequate ripple

current rating. The RMS input current is:

IIN2(RMS) =IOUT2 •

VOUT2 VIN2 –VOUT2

()

VIN2

IOUT2

and is largest when VIN2 = 2 • VOUT2 (50% duty cycle).

Considering that the maximum load current is ~1.5A, RMS

ripple current will always be less than 0.75A.

The high frequency of the LT3570 reduces the energy

storage requirements of the input capacitor, so that the

capacitance required is often less than 10μF. The combi-

nation 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 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

requirements. Because the input capacitor is likely to see

high surge currents when the input source is applied,

tantalum capacitors should be surge rated. The manu-

facturer may also recommend operation below the rated

voltage of the capacitor. Be sure to place the 1μF ceramic

as close as possible to the VIN2 and GND pins on the IC

for optimal noise immunity.

A ﬁnal caution is in order 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 LT3570. The

solution is to either clamp the input voltage or dampen the

tank circuit by adding a lossy capacitor in parallel with the

ceramic capacitor. For details, see Application Note 88.

Boost Input Capacitor Selection

The capacitor of a boost converter is less critical due to

the fact that the input current waveform is triangular and

does not contain large squarewave currents as found in

the output capacitor. Capacitors in the range of 10μF to

100μF with an ESR of 0.3Ω or less work well up to the

full 1.5A switch current. Higher ESR capacitors may be

acceptable at low switch currents. Input capacitor ripple

current for boost converters is:

IRIPPLE = 0.3 • VIN1 •

VOUT1 –VIN1

f•L • VOUT1

Buck Diode Selection

The catch diode (D2 from Figure 1) conducts current only

during switch off time. Average forward current in normal

operation can be calculated from:

ID(AVG) =IOUT1 •

VIN1 –VOUT1

VIN1

The only reason to consider a diode with a larger current

rating than necessary for nominal operation is for the

worst-case condition of shorted output. The diode current

will then increase to the typical peak switch current.

Peak reverse voltage is equal to the regulator input voltage.

Use a diode with a reverse voltage rating greater than the

input voltage. Table 3 lists several Schottky diodes and

their manufacturers.

Table 3. Schottky Diodes

PART NUMBER

VR (V)

IAVE (A)

VF AT 1A (mV)

On Semiconductor

MBRM120E

530

MBRM140

550

Diodes Inc.

B120

500

B130

500

International Rectiﬁer

10BQ030

420

APPLICATIONS INFORMATION

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