LT8335 datasheet(16/34 Pages) AD

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LT8357

Rev. 0

For more information www.analog.com

Loop Compensation

The LT8357 uses an internal transconductance error ampli-

fier, the output of which, VC, compensates the control loop.

The external inductor, output capacitor, and the compensa-

tion resistor and capacitor determine the loop stability.

The inductor and output capacitor are chosen based on

performance, size and cost. The compensation resistor

and capacitor on the VC pin are set to optimize control

loop response and stability. For a typical application, a

2.2nF compensation capacitor on the VC pin is adequate,

and a series resistor should always be used to increase

the slew rate on the VC pin to maintain tight output voltage

regulation during fast transients.

APPLICATION CIRCUITS

The LT8357 can be configured in different topologies. The

first topology to be analyzed will be the boost converter,

followed by the flyback and SEPIC converters.

Boost Converter: Switch Duty Cycle and Frequency

The LT8357 can be configured as a boost converter for

applications where the converter output voltage is higher

than the input voltage. Remember that boost convert-

ers are not short-circuit protected. Under a shorted out-

put condition, the inductor current is limited only by the

input supply capability. For applications requiring a step-

up converter that is short-circuit protected, please refer

to the Applications Information section covering SEPIC

converters.

The conversion ratio as a function of duty cycle is

VOUT

VIN

1−D

in continuous conduction mode (CCM).

For a boost converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT) and the input voltage (VIN). The maximum

APPLICATIONS INFORMATION

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 but at the cost of

reduced efficiencies and higher switching currents.

Boost Converter: Inductor and Sense Resistor

Selection

For the boost topology, the maximum average inductor

current is:

IL(MAX) =IO(MAX)•

1−DMAX

Then, the ripple current can be calculated by:

ΔIL = χ•IL(MAX) = χ•IO(MAX)•

1−DMAX

The constant χ in the preceding equation represents the

percentage peak-to-peak ripple current in the inductor,

relative to IL(MAX).

The inductor ripple current has a direct effect on the

choice of the inductor value. Choosing smaller values of

ΔIL requires large inductances and reduces the current

loop gain (the converter will approach voltage mode).

Accepting larger values of ΔIL provides fast transient

response and allows the use of low inductances, but

results in higher input current ripple and greater core

losses. It is recommended that χ fall within the range of

0.2 to 0.6.

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:

L =

VIN(MIN)

ΔIL •f

•DMAX

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