LTC1871 datasheet(13/36 Pages) LINER

13 / 36 page

LTC1871

APPLICATIO S I FOR ATIO

Application Circuits

A basic LTC1871 application circuit is shown in

Figure 1. External component selection is driven by the

characteristics of the load and the input supply. The first

topology to be analyzed will be the boost converter,

followed by SEPIC (single ended primary inductance

converter).

Boost Converter: Duty Cycle Considerations

For a boost converter operating in a continuous conduc-

tion mode (CCM), the duty cycle of the main switch is:









–

where VD is the forward voltage of the boost diode. For

converters where the input voltage is close to the output

voltage, the duty cycle is low and for converters that

develop a high output voltage from a low voltage input

supply, the duty cycle is high. The maximum output

voltage for a boost converter operating in CCM is:

OMAX

IN MIN

MAX

()

–

= ()

The maximum duty cycle capability of the LTC1871 is

typically 92%. This allows the user to obtain high output

voltages from low input supply voltages.

Boost Converter: The Peak and Average Input Currents

The control circuit in the LTC1871 is measuring the input

current (either by using the RDS(ON) of the power MOSFET

or by using a sense resistor in the MOSFET source), so the

output current needs to be reflected back to the input in

order to dimension the power MOSFET properly. Based on

the fact that, ideally, the output power is equal to the input

power, the maximum average input current is:

The peak input current is

IN MAX

OMAX

MAX

IN PEAK

O MAX

MAX

()

–

•

–









The maximum duty cycle, DMAX, should be calculated at

minimum VIN.

Boost Converter: Ripple Current

∆IL and the ‘χ’ Factor

The constant ‘χ’ in the equation above represents the

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

relative to its maximum value. For example, if 30% ripple

current is chosen, then χ = 0.30, and the peak current is

15% greater than the average.

For a current mode boost regulator operating in CCM,

slope compensation must be added for duty cycles above

50% in order to avoid subharmonic oscillation. For the

LTC1871, this ramp compensation is internal. Having an

internally fixed ramp compensation waveform, however,

does place some constraints on the value of the inductor

and the operating frequency. If too large an inductor is

used, the resulting current ramp (

∆IL)willbesmallrelative

to the internal ramp compensation (at duty cycles above

50%), and the converter operation will approach voltage

mode (ramp compensation reduces the gain of the current

loop). If too small an inductor is used, but the converter is

still operating in CCM (near critical conduction mode), the

internal ramp compensation may be inadequate to prevent

subharmonic oscillation. To ensure good current mode

gain and avoid subharmonic oscillation, it is recom-

mended that the ripple current in the inductor fall in the

range of 20% to 40% of the maximum average current. For

example, if the maximum average input current is 1A,

choose a

∆IL between 0.2A and 0.4A, and a value ‘χ’

between 0.2 and 0.4.

Boost Converter: Inductor Selection

Given an operating input voltage range, and having chosen

the operating frequency and ripple current in the inductor,

the inductor value can be determined using the following

equation:

where

IN MIN

MAX

OMAX

MAX

∆

∆=

()

•

–

LTC1871 Datasheet(PDF) 13 Page - Linear Technology

LTC1871 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL