Data Sheet

LT8349

analog.com

Rev B

21 of 29

Figure 30. A Simplified LT8349 Power Stage with a Diode Added Between VIN and VOUT

Inductor Selection

When operating in continuous conduction mode (CCM), the duty cycle can be calculated based on the output

voltage (VOUT) and the input voltage (VIN). The maximum duty cycle (DMAX)occurs when the converter has the

minimum input voltage:

DMAX =

VOUT − VIN(MIN)

VOUT

Discontinuous conduction mode (DCM) provides higher conversion ratios at a given frequency at the cost of reduced

efficiencies and higher switching currents.

The ripple current ΔISW of each inductor has a direct effect on the choice of the inductor value, the converter’s

maximum output current capability, and the light load efficiency in Burst Mode operation. Choosing smaller values

of ΔISW increases output current capability and improves light load efficiency in Burst Mode operation, but require

large inductance values and reduce the current loop gain. Accepting larger values of ΔISW provides fast transient

response and allows the use of low inductance values, but results in higher input current ripple, greater core losses,

lower light load efficiency in Burst Mode operation, and lower output current capability. Large values of ΔISW at high

duty cycle operation may result in sub-harmonic oscillation. ΔISW= 1.2A to 2A generally provides a good starting value

for many applications, and careful evaluation of system stability should be made to ensure adequate design margin.

Given an operating input voltage range, and having chosen the operating frequency and ripple current in each of the

inductors, each inductor value of the boost converter can be determined using the following Equation:

L1 = L2 =

VIN(MIN)

∆ISW • fSW

• DMAX

The peak current of each inductor is equal to the LT8349 bottom switch current limit as given in the Electrical

Characteristics table. The user should choose an inductor with sufficient saturation and RMS current ratings to

handle the inductor’s peak current.

Input Capacitor Selection

The input ripple current in a boost converter is relatively low (compared with the output ripple current), because this

current is continuous. The value of CIN is a function of the source impedance, and in general, the higher the source

impedance, the higher the required input capacitance. The RMS CIN ripple current can be estimated by the following

equation:

IRMS(CIN) = 0.3 • ∆IL

D

L1

L2

COUT

CIN

VOUT

VIN

SW1

SW2

VIN

VOUT

GND

LT8349