LT8607 datasheet(15/24 Pages) AD

15 / 24 page

LT8609S

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

Input Capacitor

Bypass the input of the LT8609S circuit with a ceramic

capacitor of X7R or X5R type. Y5V types have poor per-

formance over temperature and applied voltage, and

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

is adequate to bypass the LT8609S and will easily handle

the ripple current. 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 sig-

nificant 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 rip-

ple at the LT8609S 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 LT8609S (see the PCB Layout sec-

tion). A second precaution regarding the ceramic input

capacitor concerns the maximum input voltage rating of

the LT8609S. A ceramic input capacitor combined with

trace or cable inductance forms a high quality (under

damped) tank circuit. If the LT8609S circuit is plugged

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

nominal value, possibly exceeding the LT8609S’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 LT8609S 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 LT8609S’s control loop. Ceramic capacitors

have very low equivalent series resistance (ESR) and pro-

vide 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

capacitor placed between VOUT and FB. Increasing the out-

put capacitance will also decrease the output voltage rip-

ple. 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

Ceramiccapacitorsaresmall,robustandhaveverylowESR.

However, ceramic capacitors can cause problems when

used with the LT8609S due to their piezoelectric nature.

When in Burst Mode operation, the LT8609S’s switching

frequency depends on the load current, and at very light

loads the LT8609S can excite the ceramic capacitor at audio

frequencies, generating audible noise. Since the LT8609S

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 LT8609S. As

previously mentioned, a ceramic input capacitor com-

bined with trace or cable inductance forms a high quality

(under damped) tank circuit. If the LT8609S circuit is

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

twice its nominal value, possibly exceeding the LT8609S’s

rating. This situation is easily avoided (see Analog Devices

Application Note 88).

Enable Pin

The LT8609S 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

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