Datasheet

12
LT1680
S1 + S
X
is greater than S2, this condition for subharmonic
oscillation no longer exists.
For boost topologies, the required additional current wave-
form slope, or “Slope Compensation,” follows the relation:
S
SDC
DC
X
≥
()( )
()
12 1
1
–
–
For duty cycles less than 50% (DC < 0.5), S
X
is negative and
is not required. For duty cycles greater than 50%, S
X
takes
on values dependent on S1 and duty cycle. S1 is simply V
IN
/
L. This leads to a minimum inductance requirement for a
given V
IN
, duty cycle and slope compensation (S
X
) of:
L
V
S
DC
DC
MIN
IN
X
=






()
21
1
–
–
The LT1680 contains an internal slope compensation
ramp that has an equivalent current referred value of:
S = 0.084
f
R
X
O
SENSE






Amp/s
where f
O
is oscillator frequency and R
SENSE
is the external
current sense resistor. This yields a minimum inductance
requirement of:
L
VR DC
fDC
MIN
IN SENSE
O
≥
()()()
()()
−
()
[]
21
0 084 1
–
.
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.
OSCILLATOR
PERIOD
TIME
0 0
5a 5b
∆I
T1
I
2
I
1
S1 S1S2 S2
S1 + S
X
1680 F05
Figure 5. Inductor Current at DC > 50% and
Slope Compensation Adjusted Signal
APPLICATIO S I FOR ATIO
WUUU
Figure 6. Maximum Peak Ripple Current (Normalized)
vs Duty Cycle for Average Current Limit
DUTY CYCLE
0
1.10
MAXIMUM PEAK RIPPLE CURRENT (I
PK
/I
AVG
)
1.15
1.25
1.30
1.35
0.2 0.3 0.4 0.5 0.6 0.7 0.8
1680 F06
1.20
0.1 0.9
1.40
1.45
If an inductor smaller than the minimum required for
internal slope compensation (calculated above as L
MIN
) is
desired, additional slope compensation is necessary. The
LT1680 provides this capability through the SL/ADJ pin.
This feature is implemented by referencing this pin via a
resistor divider from the 5V
REF
pin to ground. The addi-
tional slope compensation will be affected at the point in
the oscillator waveform (at pin C
T
) corresponding to the
voltage set by the resistor divider. Additional slope com-
pensation can be calculated using the relation:
S
f
RR
X
O
TH SENSE
=
()()
()( )
2500
Amp/s
where R
TH
is the Thevenin resistance of the resistor
divider. Actual compensation will actually be somewhat
greater due to internal curvature correction circuitry that