Datasheet
13
LT1339
sn1339 1339fas
APPLICATIONS INFORMATION
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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 S
X
) as shown in Figure 5b. If the sum of the slopes
S1 + S
X
is greater than S2, the condition for subharmonic
oscillation no longer exists.
For a buck converter, the required additional current wave-
form slope, or “Slope Compensation,” follows the relation:
S
V
L
DC
X
IN
≥
−
()
21
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. This leads
to a minimum inductance requirement for a given V
IN
and
duty cycle of:
L
V
S
DC
MIN
IN
X
=
−
()
21
The LT1339 contains an internal S
X
slope compensation
ramp that has an equivalent current referred value of:
0.084
f
R
O
SENSE
Amp/s
where f
O
is oscillator frequency. This yields a minimum
inductance requirement of:
L
VR DC
f
MIN
IN SENSE
O
≥
()( )
−
()
()()
21
0 084.
A down side of slope compensation is that, since the IC servo
loop senses an increase in perceived inductor current, 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 com-
pensation. The LT1339, however, uses a current limit
scheme that is independent of slope compensation effects
(average current limit). 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 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 L
MIN
) 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 5V
REF
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:
S
f
RR
XADD
O
EQ SENSE
=
()()
()( )
2500
Amp/s
where R
EQ
is the effective resistance of the resistor divider.
Actual compensation will be somewhat greater due to in-
ternal curvature correction circuitry that imposes an expo-
nential increase in the slope compensation waveform,
OSCILLATOR
PERIOD
TIME
0 0
ab
∆I
T1
I
2
I
1
S1 S1S2 S2
S1 + S
X
1339 • F05
Figure 5. Inductor Current at DC > 50% and
Slope Compensation Adjusted Signal
DUTY CYCLE (DC)
0
PEAK/AVG
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