LT8331 datasheet(17/24 Pages) AD

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LT8337/LT8337-1

Rev. 0

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Discontinuous conduction mode (DCM) provides higher

conversion ratios at a given frequency at the cost of

reduced efficiencies and higher switching currents.

The inductor ripple current ∆ISW 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 ∆ISW

increases output current capability and light load effi-

ciency in Burst Mode operation, but require large induc-

tance values and reduce the current loop gain. Accepting

larger values of ∆ISW 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 ∆ISW at high duty cycle

operation may result in sub-harmonic oscillation. ∆ISW =

1.2A to 2.4A generally provides a good starting value for

many applications, and careful evaluation of system sta-

bility should be made to ensure adequate design margin.

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 Equation 11.

L =

VIN(MIN)

∆ISW • fSW

•DMAX

(11)

The peak inductor current is equal to the LT8337/

LT8337-1 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 voltage rating of the input

capacitor, CIN, should comfortably exceed the maximum

input voltage. Although ceramic capacitors can be rela-

tively tolerant of overvoltage conditions, aluminum elec-

trolytic capacitors are not. Be sure to characterize the

APPLICATIONS INFORMATION

input voltage for any possible overvoltage transients that

could apply excess stress to the input capacitors.

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

Equation 12.

IRMS(CIN) = 0.3 • ∆IL

(12)

Output Capacitor Selection

The output capacitor has two essential functions. First, it

filters the LT8337/LT8337-1’s discontinuous top switch

current to produce the DC output. In this role, it deter-

mines the output ripple, thus low impedance at the switch-

ing frequency is important. The second function is to store

energy in order to satisfy transient loads and stabilize the

IC’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 out-

put capacitance and the addition of a feedforward capaci-

tor placed between VOUT and FB. When a feedforward

capacitor is used or output capacitance is adjusted, a

careful evaluation of system stability should be made to

ensure adequate design margin. Increasing the output

capacitance will also decrease the output voltage ripple.

Lower value of output capacitance can be used to save

space and cost, but transient performance will suffer and

loop instability may result.

Besides the bulk output capacitors, two small output

ceramic capacitors, 1µF each, should be placed as close

as possible to the IC to complete the Silent Switcher can-

cellation loops.

See the Board Layout section for more detail. XR7 or

X5R capacitors are recommended for best performance

across temperature and output voltage variations. Note

that larger output capacitance is required when a lower

switching frequency is used. If there is significant induc-

tance to the load due to long wires or cables, additional

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