LTC4002 datasheet(14/24 Pages) LINER

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LTC3783

3783f

Figure 6 illustrates these various quantities in relation to

one another.

Typically, in order to avoid visible flicker, fPWM should be

greater than 120Hz. Assuming inductor and capacitor

sizing which is close to discontinuous operation, 2 fOSC

cycles are sufficient for proper PWM operation. Thus,

within the 1MHz rated maximum fOSC, a dimming ratio of

1/DPWM = 3000 is possible.

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:

IN MAX

OUT MAX

MAX

()

–

The peak input current is:

IN PEAK

OUT 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 = 0.3, 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

LTC3783, 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 to 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

∆ILbetween0.2Aand0.4A,andcorrespondingly

a value ‘χ’ between 0.2 and 0.4.

OPERATIO

Figure 6. PWM Dimming Parameters

PWMIN

GATE

3783 F06

# = N

DPWM/fPWM

1/fPWM

1/fOSC

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:

OUT

–

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 input voltage, the

duty cycle is high. The maximum output voltage for a

boost converter operating in CCM is:

OUT MAX

IN MIN

MAX

()

–

The maximum duty cycle capability of the LTC3783 is

typically 90%. 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 LTC3783 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

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