LT8335 datasheet(14/20 Pages) AD

14 / 20 page

LT8338

Rev. 0

For more information www.analog.com

TYPICAL APPLICATIONS

benefits, the trade-off is higher inductor peak current,

lower available output power and reduced efficiency.

Inductor Selection

The inductor peak-to-peak current ripple ∆ISW has a direct

effect on the choice of the inductor value, the converter’s

maximum output current capability, and the light load effi-

ciency in Burst Mode operation. Smaller values of ∆ISW

increase output current capability and light load efficiency

in Burst Mode operation, but require large inductances

and reduces the current loop gain (the converter will

approach voltage mode). Larger values of ∆ISW provide

fast transient response and allow the use of low induc-

tances, but result in higher input current ripple and greater

core losses, reduce the light load efficiency in Burst Mode

operation, and reduce output current capability.

Given an operating input voltage range, and having cho-

sen the operating frequency and ripple current in the

inductor, the inductor value of the boost converter can

be determined using the following equation:

VIN(MIN)

∆ISW • ƒ

• DMAX

(4)

here the ripple current ∆ISW can be set to 0.2A as a good

starting point. The peak inductor current is the switch

current limit (1.2A typical). The user should choose an

inductor having a sufficient saturation and RMS current

rating, and a low DCR to minimize I2R power losses.

Input Capacitor Selection

Bypass the input of the LT8338 circuit with a ceramic

capacitor of type X7R or X5R. The value of the input bypass

capacitor is a function of the source impedance, and in

general, the higher the source impedance, the higher the

required input capacitance. The capacitor value depends

on the input current ripple as well. The input ripple current

in a boost converter is relatively low (compared with the

output ripple current) because this current is continuous.

A 2.2μF to 10μF ceramic capacitor is adequate to bypass

the LT8338 and will easily handle the ripple current. 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.

The voltage rating of the input capacitor, C1, should com-

fortably exceed the maximum input voltage. Although

ceramic capacitors can be relatively tolerant of overvolt-

age conditions, aluminum electrolytic capacitors are not.

Be sure to characterize the input voltage for any possible

overvoltage transients that could apply excess stress to

the input capacitors.

Output Capacitor Selection

The output capacitor has two essential functions. First, it

filters LT8338’s discontinuous top switch current to pro-

duce the DC output. In this role it determines the output

ripple, and 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 LT8338’s

control loop. The X5R or X7R type ceramic capacitors

have very low equivalent series resistance (ESR), which

provides low output ripple and good transient response.

Transient performance can be improved with higher value

output capacitance. Increasing the output capacitance will

also decrease the output voltage ripple. Lower values of

output capacitance can be used to save space and cost

but transient performance will suffer and may cause

loop instability.

A 4.7μF ceramic capacitor is adequate for the LT8338 out-

put capacitor. This ceramic should be placed near to VOUT/

GND. See the Board Layout section for more details. Note

that larger output capacitance is required when a lower

switching frequency is used. When choosing a capacitor,

special attention should be given to capacitor’s data sheet

to calculate the effective capacitance under the relevant

operating conditions of voltage bias and temperature. A

physically larger capacitor or one with a higher voltage

rating may be required. For good starting values, refer to

the Typical Application section.

Board Layout

Figure 4 shows a recommended PCB layout. For more

details and PCB design files refer to the Demo Board guide

for the LT8338.

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