lt1339 Linear Technology Corporation, lt1339 Datasheet - Page 13

• Manufacturer Part Number: lt1339
• Description: High Power Synchronous Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
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
S1 + S
oscillation no longer exists.
For a buck converter, the required additional current wave-
form slope, or “Slope Compensation,” follows the relation:
For duty cycles less than 50% (DC < 0.5), S
and is not required. For duty cycles greater than 50%, S
takes on values dependent on S1 and duty cycle. This leads
to a minimum inductance requirement for a given V
duty cycle of:
The LT1339 contains an internal S
ramp that has an equivalent current referred value of:
where f
inductance requirement of:
A down side of slope compensation is that, since the IC servo
loop senses an increase in perceived inductor current, the
I
1
0
L
L
S
0.084
MIN
MIN
X
X
I
2
X
O
is greater than S2, the condition for subharmonic
) as shown in Figure 5b. If the sum of the slopes
is oscillator frequency. This yields a minimum
OSCILLATOR
Figure 5. Inductor Current at DC > 50% and
Slope Compensation Adjusted Signal
V
S1
R
L
IN
PERIOD
V
SENSE
I
V
S
IN
IN
f
X
O
2
T1
R
DC
2
0 084
S2
SENSE
DC
U
a
1
1
INFORMATION
f
U
2
O
DC
TIME
Amp/s
1
X
W
slope compensation
S1 + S
X
S1
is negative
X
U
b
IN
1339 • F05
S2
and
0
X
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
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
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:
where R
Actual compensation will be somewhat greater due to in-
ternal curvature correction circuitry that imposes an expo-
nential increase in the slope compensation waveform,
S
XADD
Figure 6. Maximum Ripple Current (Normalized)
vs Duty Cycle for Average Current Limit
EQ
is the effective resistance of the resistor divider.
1.45
1.40
1.35
1.30
1.25
1.20
1.15
1.10
R
0
EQ
2500
0.1
R
0.2
SENSE
f
O
0.3
DUTY CYCLE (DC)
0.4
REF
0.5
Amp/s
pin to ground. The addi-
0.6
0.7 0.8 0.9
LT1339 • F06
LT1339
sn1339 1339fas
13
MIN
) is