LT8350RVPBF datasheet(18/24 Pages) AD

18 / 24 page

LT8350

Rev. 0

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oscillations at certain duty cycles. The minimum induc-

tance required for stability can be calculated as:

L >

VOUT

2 • fSW • ISW(MAX)

where:

fSW is switching frequency

ISW(MAX) is maximum switch current limit = 6.8A (typical)

For high efficiency, choose an inductor with low core loss,

such as ferrite. Also, the inductor should have low DC

resistance to reduce the I2R losses, and must be able to

handle the peak inductor current without saturating. To

minimize radiated noise, use a shielded inductor.

CIN and COUT Selection

Input and output capacitance is necessary to suppress

voltage ripple caused by discontinuous current moving

in and out the regulator. A parallel combination of capaci-

tors is typically used to achieve high capacitance and low

equivalent series resistance (ESR). Dry tantalum, special

polymer, aluminum electrolytic and ceramic capacitors are

all available in surface mount packages. Capacitors with

low ESR and high ripple current ratings, such as OS-CON

and POSCAP are also available.

Ceramic capacitors should be placed near the regulator

input and output to suppress high frequency switching

spikes. Ceramic capacitors, of at least 1µF, should also

be placed from VIN to GND and VOUT to GND as close

to the LT8350 pins as possible. Due to their excellent

low ESR characteristics, ceramic capacitors can signifi-

cantly reduce input ripple voltage and help reduce power

loss in the higher ESR bulk capacitors. X5R or X7R

dielectrics are preferred, as these materials retain their

capacitance over wide voltage and temperature ranges.

Many ceramic capacitors, particularly 0805 or 0603 case

sizes, have greatly reduced capacitance at the desired

operating voltage.

Input Capacitance CIN

Discontinuous input current is highest in the buck region

due to the switch A toggling on and off. Make sure that the

CIN capacitor network has low enough ESR and is sized

to handle the maximum RMS current. In buck region, the

input RMS current is given by:

IRMS ≈

–1

IOUT(MAX) •

•

VOUT

VIN

VOUT

The formula has a maximum at VIN = 2VOUT, where IRMS

= IOUT(MAX)/2. This simple worst-case condition is com-

monly used for design because even significant deviations

do not offer much relief.

Output Capacitance COUT

Discontinuous current shifts from the input to the output

in the boost region. Make sure that the COUT capacitor

network is capable of reducing the output voltage ripple.

The effects of ESR and the bulk capacitance must be con-

sidered when choosing the right capacitor for a given

output ripple voltage. The maximum steady state ripple

due to charging and discharging the bulk capacitance is

given by:

∆VCAP(BOOST) =

∆VCAP(BUCK) =

IOUT • (VOUT – VIN(MIN))

COUT • VOUT • fSW

VOUT

VIN(MIN)

VOUT • (1 –

)

8 • L • fSW • COUT

The maximum steady ripple due to the voltage drop

across the ESR is given by:

∆VERS(BOOST) =

• ESR

∆VERS(BUCK) =

VOUT • IOUT(MAX)

VIN(MIN)

VOUT

VIN(MIN)

VOUT • (1 –

)

L • fSW

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