LTC1872 datasheet(14/36 Pages) LINER

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LTC1871

1871fe

APPLICATIONS INFORMATION

Boost Converter: Ripple Current ΔIL 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 (ΔIL) will be small relative

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 ΔIL 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:

VIN(MIN)

IL • f

•DMAX

where:

IL = •

IO(MAX)

1– DMAX

Remember that boost converters are not short-circuit

protected. Under a shorted output 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 section

covering SEPIC converters.

The minimum required saturation current of the inductor

can be expressed as a function of the duty cycle and the

load current, as follows:

IL(SAT)

•

IO(MAX)

1– DMAX

The saturation current rating for the inductor should be

checked at the minimum input voltage (which results

in the highest inductor current) and maximum output

current.

Boost Converter: Operating in Discontinuous Mode

Discontinuous mode operation occurs when the load cur-

rent is low enough to allow the inductor current to run out

during the off-time of the switch, as shown in Figure 9.

Once the inductor current is near zero, the switch and diode

capacitances resonate with the inductance to form damped

ringing at 1MHz to 10MHz. If the off-time is long enough,

the drain voltage will settle to the input voltage.

Depending on the input voltage and the residual energy

in the inductor, this ringing can cause the drain of the

power MOSFET to go below ground where it is clamped

by the body diode. This ringing is not harmful to the IC

and it has not been shown to contribute signiﬁcantly to

EMI. Any attempt to damp it with a snubber will degrade

the efﬁciency.

Figure 9. Discontinuous Mode Waveforms

MOSFET DRAIN

VOLTAGE

2V/DIV

INDUCTOR

CURRENT

2A/DIV

2μs/DIV

1871 F09

VIN = 3.3V IOUT = 200mA

VOUT = 5V

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