LT1339C datasheet(13/20 Pages) LINER

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LT1339

APPLICATIONS INFORMATION

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. This leads

to a minimum inductance requirement for a given VIN and

duty cycle of:

MIN









−

()

The LT1339 contains an internal SX slope compensation

ramp that has an equivalent current referred value of:

0.084

SENSE









Amp/s

where fO is oscillator frequency. This yields a minimum

inductance requirement of:

MIN

SENSE

≥ ()(

) −

()

()( )

0 084

A down side of slope compensation is that, since the IC

servo loop senses an increase in perceived inductor cur-

rent, the internal current limit functions are affected such

that the maximum current capability of a regulator is

reduced by the same amount as the effective current

referred slope compensation. The LT1339, however, uses

a current limit scheme that is independent of slope com-

pensation effects (average current limit). This provides

operation at any duty cycle with no reduction in current

sourcing capability, provided ripple current peak ampli-

tude is less than 15% of the current limit value. For

example, if the supply is set up to current limit at 10A, as

long as the peak inductor current is less than 11.5A, duty

cycles up to 90% can be achieved without compromising

the average current limit value.

If an inductor smaller than the minimum required for

internal slope compensation (calculated above as LMIN) is

desired, additional slope compensation is required. The

LT1339 provides this capability through the SL/ADJ pin.

This feature is implemented by referencing this pin via a

resistor divider from the 5VREF pin to ground. The addi-

tional slope compensation will be affected at the point in

the oscillator waveform (at pin CT) corresponding to the

voltage set by the resistor divider. Additional slope com-

pensation can be calculated using the relation:

XADD

SENSE

= ()( )

()(

)

2500

Amp/s

where REQ is the effective resistance of the resistor divider.

Actual compensation will be somewhat greater due to

internal curvature correction circuitry that imposes an

exponential increase in the slope compensation wave-

form, further increasing the effective compensation slope

up to 20% for a given setting.

OSCILLATOR

PERIOD

TIME

∆I

S1 + SX

1339 • F05

Figure 5. Inductor Current at DC > 50% and

Slope Compensation Adjusted Signal

DUTY CYCLE (DC)

0.4

1.45

1.40

1.35

1.30

1.25

1.20

1.15

1.10

LT1339 • F06

0.2

0.6

0.1

0.5

0.3

0.7 0.8 0.9

Figure 6. Maximum Ripple Current (Normalized)

vs Duty Cycle for Average Current Limit

Design Example:

VIN = 20V

VOUT = 15V (DC = 0.75)

RSENSE = 0.01Ω

fO = 100kHz

L = 5

µH

The minimum inductor usable with no additional slope

compensation is:

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