LT8607 datasheet(15/24 Pages) AD

15 / 24 page

LT8607/LT8607B

Rev. D

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APPLICATIONS INFORMATION

significant inductance due to long wires or cables, addi-

tional 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 rip-

ple at the LT8607 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 LT8607 (see the PCB Layout section).

A second precaution regarding the ceramic input capaci-

tor concerns the maximum input voltage rating of the

LT8607. A ceramic input capacitor combined with trace

or cable inductance forms a high quality (under damped)

tank circuit. If the LT8607 circuit is plugged into a live

supply, the input voltage can ring to twice its nominal

value, possibly exceeding the LT8607’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

with the inductor, it filters the square wave generated

by the LT8607 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 sta-

bilize the LT8607’s control loop. Ceramic capacitors have

very low equivalent series resistance (ESR) and provide

the best ripple performance. A good starting value is:

COUT =

100

VOUT • fSW

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

output capacitor and the addition of a feedforward capaci-

tor 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 and may

cause loop instability. See the Typical Applications in this

data sheet for suggested capacitor values.

When choosing a capacitor, special attention should be

given to the data sheet to calculate the effective capaci-

tance under the relevant operating conditions of voltage

bias and 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 LT8607 due to their piezoelectric

nature. When in Burst Mode operation, the LT8607’s

switching frequency depends on the load current, and at

very light loads the LT8607 can excite the ceramic capacitor

at audio frequencies, generating audible noise. Since the

LT8607 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 LT8607. As pre-

viously mentioned, a ceramic input capacitor combined

with trace or cable inductance forms a high quality (under

damped) tank circuit. If the LT8607 circuit is plugged into

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

nal value, possibly exceeding the LT8607’s rating. This

situation is easily avoided (see Analog Devices Application

Note 88).

Enable Pin

The LT8607 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

LT8607 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

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