LT8697_15 datasheet(21/30 Pages) LINER

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LT8697

8697fb

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APPLICATIONS INFORMATION

switching frequency and maximum input voltage used in

the desired application. Note that the LT8697 peak switch

current decreases in the 125°C to 150°C H-grade junction

temperature range. The maximum output current that the

LT8697 can deliver at 150°C junction temperature and

maximum duty cycle may be less than 2.5A depending

on the inductor value.

The optimum inductor for a given application may differ

from the one indicated by this design guide. A larger value

inductor provides a higher maximum load current and

reduces the output voltage ripple. For applications requir-

ing smaller load currents, the value of the inductor may

be lower and the LT8697 may operate with higher ripple

current. This allows use of a physically smaller inductor,

or one with a lower DCR resulting in higher efficiency.

For more information about 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 LMIN is required to avoid

sub-harmonic oscillation:

LMIN =

5.8V

+ VSW(BOT)

fSW

• 0.8

For robust operation over a wide VIN and VOUT range, use

at least an inductor value as specified above.

Input Capacitor

Bypass the input of the LT8697 circuit with a ceramic ca-

pacitor of X7R or X5R type placed as close as possible to

the VIN and PGND pins. 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

bypasstheLT8697andwilleasilyhandletheripplecurrent.

Note that larger input capacitance is required when a lower

switching frequency is used. 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 LT8697 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 LT8697 (see the PCB Layout section).

Asecondprecautionregardingtheceramicinputcapacitor

concernsthemaximuminputvoltageratingoftheLT8697.

A ceramic input capacitor combined with trace or cable

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

cuit. If the LT8697 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the LT8697’s voltage rating. This situation is

easilyavoided(seeLinearTechnologyApplicationNote88).

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated

by the LT8697 to produce the DC output. In this role it

determines the output ripple, thus low impedance at the

switching frequency is important. The second function

is to store energy in order to satisfy transient loads and

stabilize the LT8697’s control loop. Ceramic capacitors

have very low equivalent series resistance (ESR) and

provide the best ripple performance. For good starting

values, see the Typical Applications section.

Use X5R or X7R types. This choice will provide low output

ripple and good transient response. 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 this may cause loop instability if the output

capacitor is too small. Since cable drop compensation

slews the voltage across the output capacitor in response

to transient load steps, a smaller output capacitor can give

faster response time. See the Typical Applications in this

data sheet for suggested capacitor values.

When choosing a capacitor, special attention should be

giventothedatasheettocalculatetheeffectivecapacitance

undertherelevantoperatingconditionsofvoltagebiasand

temperature. A physically larger capacitor or one with a

higher voltage rating may be required.

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