LT8604 datasheet(13/20 Pages) AD

13 / 20 page

LT8604

Rev. 0

For more information www.analog.com

ISAT) rating of the inductor must be higher than the load

current plus 1/2 of the inductor ripple current:

IL(PEAK) =ILOAD(MAX) +

ΔL

where ∆IL is the inductor ripple current as calculated sev-

eral paragraphs below and ILOAD(MAX) is the maximum

output load for a given application.

As a quick example, an application requiring 120mA out-

put should use an inductor with an RMS rating of greater

than 120mA and an ISAT of greater than 180mA. To keep

the efficiency high, the series resistance (DCR) should be

less than 1Ω, and the core material should be intended

for high frequency applications.

The LT8604 limits the peak switch current in order to

protect the switches and the system from overload faults.

The top switch current limit (ILIM) is at least 185mA at

low duty cycles and decreases linearly to 137mA at D =

0.8. The inductor value must then be sufficient to supply

the desired maximum output current (IOUT(MAX)), which

is a function of the switch current limit (ILIM) and the

ripple current:

IOUT(MAX) =ILIM –

ΔIL

The peak-to-peak ripple current in the inductor can be

calculated as follows:

ΔIL =

VOUT

L • fSW

1–

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

where fSW is the switching frequency of the LT8604, and

L is the value of the inductor. Therefore, the maximum

output current that the LT8604 will deliver depends on

the switch current limit, the inductor value, and the input

and output voltages. The inductor value may have to be

increased if the inductor ripple current does not allow

sufficient maximum output current (IOUT(MAX)) given the

switching frequency, and maximum input voltage used in

the desired application.

For more information about maximum output current and

discontinuous operation, see Analog Devices Application

Note 44.

Finally, for duty cycles greater than 50%, a minimum

inductance is required to avoid sub-harmonic oscillation:

LMIN =

VOUT + VSW(BOT)

fSW

•12.5

where fSW is the switching frequency, VOUT is the output

voltage, VSW(BOT) is the bottom switch drop (~0.14V) and

LMIN is the inductor value.

Input Capacitor

Bypass the input of the LT8604 circuit with a ceramic

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

mance over temperature and applied voltage, and should

not be used. A 1μF to 2.2μF ceramic capacitor is adequate

to bypass the LT8604 and will easily handle the ripple

current. 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 rip-

ple at the LT8604 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

A 1μF capacitor is capable of this task, but only if it is

placed close to the LT8604 (see the PCB Layout section).

A second precaution regarding the ceramic input capaci-

tor concerns the maximum input voltage rating of the

LT8604. A ceramic input capacitor combined with trace

or cable inductance forms a high quality (under damped)

tank circuit. If the LT8604 circuit is plugged into a live

supply, the input voltage can ring to twice its nominal

value, possibly exceeding the LT8604’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 LT8604 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

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