LT3971 datasheet(13/24 Pages) LINER

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LT3971

3971f

For details of maximum output current and discontinuous

operation, see Linear Technology’s Application Note 44.

Finally, for duty cycles greater than 50% (VOUT/VIN>0.5),

a minimum inductance is required to avoid sub-harmonic

oscillations. See Application Note 19.

One approach to choosing the inductor is to start with

the simple rule given above, look at the available induc-

tors, and choose one to meet cost or space goals. Then

use the equations above to check that the LT3971 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

Bypass the input of the LT3971 circuit with a ceramic

capacitor of X7R or X5R type. Y5V types have poor

performance over temperature and applied voltage, and

should not be used. A 4.7μF to 10μF ceramic capacitor

is adequate to bypass the LT3971 and will easily handle

the ripple current. Note that larger input capacitance is

required when a lower switching frequency is used (due

to longer on-times). If the input power source has high

impedance, or there is signiﬁcant inductance due to

long wires or cables, additional bulk capacitance may be

necessary. This can be provided with a low performance

electrolytic capacitor.

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 LT3971 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

A 4.7μF capacitor is capable of this task, but only if it is

placed close to the LT3971 (see the PCB Layout section).

A second precaution regarding the ceramic input capacitor

concerns the maximum input voltage rating of the LT3971.

A ceramic input capacitor combined with trace or cable

inductance forms a high quality (under damped) tank cir-

cuit. If the LT3971 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the LT3971’s voltage rating. This situation is

easily avoided (see the Hot Plugging Safely section).

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it ﬁlters the square wave generated by the

LT3971 to produce the DC output. In this role it determines

the output ripple, so low impedance (at the switching

frequency) is important. The second function is to store

energy in order to satisfy transient loads and stabilize the

LT3971’s control loop. Ceramic capacitors have very low

equivalent series resistance (ESR) and provide the best

ripple performance. A good starting value is:

OUT

OUT SW

100

where fSW is in MHz, and COUT is the recommended output

capacitance in μF. Use X5R or X7R types. This choice will

provide low output ripple and good transient response.

Transient performance can be improved with a higher

value capacitor. Increasing the output capacitance will

also decrease the output voltage ripple. A lower value of

output capacitor can be used to save space and cost but

transient performance will suffer.

When choosing a capacitor, look carefully through the

data sheet to ﬁnd out what the actual capacitance is under

operating conditions (applied voltage and temperature). A

physicallylargercapacitororonewithahighervoltagerating

may be required. Table 3 lists several capacitor vendors.

Table 3. Recommended Ceramic Capacitor Vendors

MANUFACTURER

WEBSITE

AVX

www.avxcorp.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

Vishay Siliconix

www.vishay.com

TDK

www.tdk.com

Catch Diode Selection

The catch diode (D1 from Block Diagram) conducts cur-

rent only during switch off time. Average forward current

in normal operation can be calculated from:

D AVG

OUT

()

–

where IOUT is the output load current. The only reason to

consider a diode with a larger current rating than necessary

APPLICATIONS INFORMATION

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