LT8646SEVPBF datasheet(22/30 Pages) AD

22 / 30 page

LT8645S/LT8646S

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

(COPT1, COPT2). These capacitors should be 0603 or 0805

in size. For automotive applications requiring 2 series

input capacitors, two small 0603 or 0805 may be placed

at each side of the LT8645S/LT8646S.

A third, larger ceramic capacitor of 4.7µF or larger should

be placed close to COPT1 or COPT2. See layout section for

more detail. X7R or X5R capacitors are recommended for

best performance across temperature and input voltage

variations.

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.

A ceramic input capacitor combined with trace or cable

inductance forms a high quality (under damped) tank

circuit. If the LT8645S/LT8646S circuit is plugged into a

live supply, the input voltage can ring to twice its nominal

value,possiblyexceedingtheLT8645S/LT8646S’svoltage

rating. This situation is easily avoided (see Analog Devices

Application Note 88).

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by

the LT8645S/LT8646S 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 LT8645S/LT8646S’s control loop. Ceramic

capacitorshaveverylowequivalentseriesresistance(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

rippleandgoodtransientresponse.Transientperformance

can be improved with a higher value output capacitor and

the addition of a feedforward capacitor placed between

VOUT and FB. 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

performancewillsufferandmaycauseloopinstability.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.

Ceramic Capacitors

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LT8645S/LT8646S due to their

piezoelectric nature. When in Burst Mode operation, the

LT8645S/LT8646S’s switching frequency depends on the

load current, and at very light loads the LT8645S/LT8646S

can excite the ceramic capacitor at audio frequencies,

generating audible noise. Since the LT8645S/LT8646S

operates at a lower current limit during Burst Mode op-

eration, the noise is typically very quiet to a casual ear. If

this is unacceptable, use a high performance tantalum or

electrolytic capacitor at the output. Low noise ceramic

capacitors are also available.

Afinalprecautionregardingceramiccapacitorsconcernsthe

maximuminputvoltageratingoftheLT8645S/LT8646S.As

previouslymentioned,aceramicinputcapacitorcombined

with trace or cable inductance forms a high quality (un-

derdamped) tank circuit. If the LT8645S/LT8646S circuit

is plugged into a live supply, the input voltage can ring to

twice its nominal value, possibly exceeding the LT8645S/

LT8646S’s rating. This situation is easily avoided (see

Analog Devices Application Note 88).

Enable Pin

The LT8645S/LT8646S is in shutdown when the EN pin is

low and active when the pin is high. The rising threshold

of the EN comparator is 1.01V, with 45mV of hysteresis.

The EN pin can be tied to VIN if the shutdown feature is not

used, or tied to a logic level if shutdown control is required.

Adding a resistor divider from VIN to EN programs the

LT8645S/LT8646S to regulate the output only when VIN is

above a desired voltage (see the Block Diagram). Typically,

thisthreshold,VIN(EN),isusedinsituationswheretheinput

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