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LT1680

High Power DC/DC Step-Up Controller

Linear Technology 

LT1680

APPLICATIONS INFORMATION

SX

=



0.084

fO

RSENSE





Amp/s

where fO is oscillator frequency and RSENSE is the external

current sense resistor. This yields a minimum inductance

requirement of:

[ ] LMIN

≥

(VIN)(RSENSE)(2DC –1)

(0.084)(fO)(1− DC)

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 LT1680, however, uses

a current limit scheme that is independent of the slope

compensation effects (Average Current Limiting). This

provides operation at any duty cycle with no reduction in

current sourcing capability, provided ripple current peak

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

example, if the converter is set up to average 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 necessary. The

LT1680 provides this capability through the SL/ADJ pin.

1.45

1.40

1.35

1.30

1.25

1.20

1.15

1.10

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

DUTY CYCLE

1680 F10

Figure 10. Maximum Peak Ripple Current (Normalized)

vs Duty Cycle for Average Current Limit

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:

( ( )( )( ) ) SX =

2500 fO

RTH RSENSE

Amp/s

where RTH is the Thevenin resistance of the resistor

divider. Actual compensation will actually be somewhat

greater due to internal curvature correction circuitry that

imposes an exponential increase in the slope compensa-

tion waveform, further increasing the effective compensa-

tion slope up to 20% for a given setting.

Design example:

VIN = 20V

VOUT = 80V (DC = 0.75)

RSENSE = 0.01Ω

fO = 100kHz

L = 20µH

The minimum inductor usable with no additional slope

compensation is:

LMIN

≥

(20V)(0.01Ω)(1.5 – 1)

(0.084)(100000)(1– 0.75)

=

47.6µH

Since L = 20µH is less than LMIN, additional slope

compensation is necessary. The total slope compensa-

tion required is:

SX

≥





20V 

20µH

(1.5

1– 0.75

–

1)

=

(2)(106 )

Amp/s

Subtracting the internally generated slope compensation

and solving for the required effective resistance at SL/ADJ

yields:

( ) ( ( )() )( )( ) REQ ≤

2500 fO

2

106





RSENSE

–

0.084

fO

= 21.5k

13

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