LTC1700_ 查看數據表（PDF） - Linear Technology

零件编号

产品描述 (功能)

生产厂家

LTC1700_

High Input Voltage, Current Mode Boost, Flyback and SEPIC Controller

Linear Technology 

LTC1871-7

APPLICATIO S I FOR ATIO

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

LTC1871-7, 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) 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 ∆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:

L

=

VIN(MIN)

∆IL • f

• DMAX

where:

∆IL

=

χ

• 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

14

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)

≥



1+

χ

2 

•

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␣ 10.

Once the inductor current is near zero, the switch and di-

ode 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.

OUTPUT

VOLTAGE

200mV/DIV

INDUCTOR

CURRENT

1A/DIV

MOSFET

DRAIN

VOLTAGE

20V/DIV

1µs/DIV

18717 F10

Figure 10. Discontinuous Mode Waveforms

for the Converter Shown in Figure 9

18717f

Share Link: