LT8608 datasheet(14/24 Pages) LINER

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LT8608/LT8608B

Rev C

For more information www.analog.com

APPLICATIONS INFORMATION

A ceramic input capacitor combined with trace or cable

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

cuit. If the LT8608 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the LT8608’s voltage 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

withtheinductor,itfiltersthesquarewavegeneratedbythe

LT8608toproducetheDCoutput.Inthisroleitdetermines

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

LT8608’s control loop. Ceramic capacitors have very low

equivalent series resistance (ESR) and provide the best

ripple performance. A good starting value is:

OUT =

100

OUT • fSW

where fSWisinMHz,andCOUTistherecommendedoutput

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 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 performance will suffer

andmaycauseloopinstability.SeetheTypicalApplications

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

whenusedwiththeLT8608duetotheirpiezoelectricnature.

When in Burst Mode operation, the LT8608’s switching

frequency depends on the load current, and at very light

loads the LT8608 can excite the ceramic capacitor at audio

frequencies, generating audible noise. Since the LT8608

operates at a lower current limit during Burst Mode

operation, 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.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT8608. As pre-

viously mentioned, a ceramic input capacitor combined

with trace or cable inductance forms a high quality (under

damped) tank circuit. If the LT8608 circuit is plugged

into a live supply, the input voltage can ring to twice its

nominal value, possibly exceeding the LT8608’s rating.

This situation is easily avoided (see Analog Devices Ap-

plication Note 88).

Enable Pin

The LT8608 is in shutdown when the EN pin is low and

active when the pin is high. The rising threshold of the EN

comparator is 1.05V, with 50mV 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

LT8608 to regulate the output only when VIN is above

a desired voltage (see Block Diagram). Typically, this

threshold, VIN(EN), is used in situations where the input

supply is current limited, or has a relatively high source

resistance. A switching regulator draws constant power

from the source, so source current increases as source

voltage drops. This looks like a negative resistance load

to the source and can cause the source to current limit or

latch low under low source voltage conditions. The VIN(EN)

threshold prevents the regulator from operating at source

voltages where the problems might occur. This threshold

can be adjusted by setting the values R3 and R4 such that

they satisfy the following equation:

IN(EN) =

+ 1

⎛

⎝

⎜

⎞

⎠

⎟ • 1V

where the LT8608 will remain off until VINisaboveVIN(EN).

Duetothecomparator’shysteresis,switchingwillnotstop

until the input falls slightly below VIN(EN).

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