Data Sheet

LT8350S

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Rev. A

28 of 36

Slope compensation provides stability in constant frequency current mode control by preventing subharmonic

oscillations at certain duty cycles. The minimum inductance required for stability can be calculated as:

L >

VOUT

2 × fSW × ISW(MAX)

where:

fSW is switching frequency

I

SW

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

For high-efficiency, choose an inductor with low core loss, such as ferrite. The inductor should also have low DC

resistance to reduce the I2R losses and must be able to handle the peak inductor current without saturating. Use a

shielded inductor to minimize radiated noise.

C

IN and COUT Selection

Input and output capacitance are necessary to suppress voltage ripple caused by discontinuous current moving in

and out of the regulator. A parallel combination of capacitors 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 significantly 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 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 (C

IN)

Discontinuous input current is highest in the buck region due to 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:

I

I_inputRMS ≈ I

OUT(MAX) ×

VOUT

VIN

× √

VIN

VOUT

- 1

The formula has a maximum at VIN = 2VOUT, where II_inputRMS = IOUT(MAX)/2. This simple worst-case condition is commonly

used for design because even significant deviations do not offer much relief.

Output Capacitance (C

OUT)

Discontinuous current shifts from the input to the output in the boost region. Make sure that the COUT capacitor

network can reduce the output voltage ripple. The effects of ESR and bulk capacitance must be considered 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:

ΔVCAP(BOOST) =

IOUT x (VOUT - VIN(MIN))

COUT x VOUT x fSW

ΔVCAP(BUCK) =

VOUT x (1 -

VOUT

VIN(MIN) )

8 x L x fSW

2 x COUT