Datasheet
R
SEN
=
I
OUT(max)
(1-D)
+
(D x V
IN
)
(2 x f
S
x L)
V
SENSE
- D x (V
SL
+'V
SL
)
DR
V
OUT
FB
L
D1
Q
I
SEN
C
OUT
R
F2
V
IN
LM3481
R
SEN
R
F1
+
R
SEN
=
V
SENSE
- (D x V
SL
)
I
OUT(max)
(1-D)
+
(D x V
IN
)
(2 x f
S
x L)
LM3481
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SNVS346E –NOVEMBER 2007–REVISED APRIL 2012
Combining the two equations yields an expression for R
SEN
Evaluate R
SEN
at the maximum and minimum V
IN
values and choose the smallest R
SEN
calculated.
Figure 31. Adjusting the Output Voltage
Current Limit with Additional Slope Compensation
If an external slope compensation resistor is used (see Figure 22) the internal control signal will be modified and
this will have an effect on the current limit.
If R
SL
is used, then this will add to the existing slope compensation. The command voltage, V
CS
, will then be
given by:
V
CS
= V
SENSE
− D x (V
SL
+ ΔV
SL
)
Where V
SENSE
is a defined parameter in the electrical characteristics section and ΔV
SL
is the additional slope
compensation generated as discussed in the Slope Compensation Ramp section. This changes the equation for
R
SEN
to:
Note that since ΔV
SL
= R
SL
x K as defined earlier, R
SL
can be used to provide an additional method for setting the
current limit. In some designs R
SL
can also be used to help filter noise to keep the I
SEN
pin quiet.
Power Diode Selection
Observation of the boost converter circuit shows that the average current through the diode is the average load
current, and the peak current through the diode is the peak current through the inductor. The diode should be
rated to handle more than the inductor peak current. The peak diode current can be calculated using the formula:
I
D(Peak)
= [I
OUT
/ (1−D)] + Δi
L
In the above equation, I
OUT
is the output current and Δi
L
has been defined in Figure 30.
The peak reverse voltage for a boost converter is equal to the regulator output voltage. The diode must be
capable of handling this peak reverse voltage. To improve efficiency, a low forward drop Schottky diode is
recommended.
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