LT1680ISW datasheet(12/16 Pages) LINER

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LT1680

APPLICATIONS INFORMATION

ments. Generally, the selection of inductor value can be

reduced to desired maximum ripple current in the inductor

(

∆I). For a boost converter, the minimum inductor value

for a given operating ripple current can be determined

using the following relation:

MIN

OUT

()

()( )( )

–

∆

Given an inductor value (L), the peak inductor current is

the sum of the average inductor current (IAVG) and half the

inductor ripple current (

∆I), or:

AVG

OUT

()

()()( )( )

–

The inductor core type is determined by peak current and

efficiency requirements. The inductor core must with-

stand this peak current without saturating, and the series

winding resistance and core losses should be kept as

small as is practical to maximize conversion efficiency.

The LT1680 peak current threshold is 40% greater than

the average limit threshold. Slope compensation effects

reduce this margin as duty cycle increases. This margin

must be maintained to prevent peak current limit from

corrupting the programmed value for average current

limit. Programming the peak ripple current to less than

15% of the desired average current limit value will assure

proper operation of the average current limit feature

through 90% duty cycle (see Slope Compensation).

Slope Compensation

Current mode switching regulators that operate with a

duty cycle greater than 50% and have continuous inductor

current can exhibit duty cycle instability. While a regulator

will not be damaged and may even continue to function

acceptably during this type of subharmonic oscillation, an

irritating high-pitched squeal is usually produced.

The criterion for current mode duty cycle instability is

met when the increasing slope of the inductor ripple

current is less than the decreasing slope, which is the

case at duty cycles greater than 50%. This condition is

illustrated in Figure 9a. The inductor ripple current starts

at I1, the beginning of each oscillator switch cycle.

Current increases at a rate S1 until the current reaches

the control trip level I2. The controller servo loop then

disables the switch and inductor current begins to de-

crease at a rate S2. If the current switch point (I2) is

perturbed slightly and increased by

∆I, the cycle time

ends such that the minimum current point is increased by

a factor of 1 + (S2/S1) to start the next cycle. On each

successive cycle, this error is multiplied by a factor of S2/

S1. Therefore, if S2/S1 is

≥1, the system is unstable.

Subharmonic oscillations can be eliminated by augment-

ing the increasing ripple current slope (S1) in the control

loop. This is accomplished by adding an artificial ramp on

the inductor current waveform internal to the IC (with a

slope SX) as shown in Figure 9b. If the sum of the slopes

S1 + SX is greater than S2, this condition for subharmonic

oscillation no longer exists.

OSCILLATOR

PERIOD

TIME

∆I

S1 + SX

1680 F09

Figure 9. Inductor Current at DC > 50% and

Slope Compensation Adjusted Signal

For boost topologies, the required additional current wave-

form slope, or “Slope Compensation,” follows the relation:

SDC

X ≥

()(

)

()

–

For duty cycles less than 50% (DC < 0.5), SX is negative and

is not required. For duty cycles greater than 50%, SX takes

on values dependent on S1 and duty cycle. S1 is simply VIN/

L. This leads to a minimum inductance requirement for a

given VIN, duty cycle and slope compensation (SX) of:

MIN







()

–

The LT1680 contains an internal slope compensation

ramp that has an equivalent current referred value of:

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