LM3481EVAL/NOPB National Semiconductor, LM3481EVAL/NOPB Datasheet - Page 15

• Manufacturer Part Number: LM3481EVAL/NOPB
• Description: EVAL BOARD FOR LM3481
• Manufacturer: National Semiconductor

Specifications of LM3481EVAL/NOPB

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Combining the two equations yields an expression for R
CURRENT LIMIT WITH ADDITIONAL SLOPE
COMPENSATION
If an external slope compensation resistor is used (see
5) the internal control signal will be modified and this will have
an effect on the current limit.
If R
sation. The command voltage, V
Where V
acteristics section and ΔV
sation generated as discussed in the Slope Compensation
Ramp section. This changes the equation for R
Note that since ΔV
used to provide an additional method for setting the current
limit. In some designs R
to keep the I
POWER DIODE SELECTION
Observation of the boost converter circuit shows that the av-
erage current through the diode is the average load current,
SL
is used, then this will add to the existing slope compen-
SENSE
SEN
is a defined parameter in the electrical char-
pin quiet.
SL
V
CS
= R
SL
= V
SL
can also be used to help filter noise
SL
SENSE
x K as defined earlier, R
is the additional slope compen-
CS
− ΔV
, will then be given by:
SL
FIGURE 14. Adjusting the Output Voltage
SEN
to:
SL
can be
Figure
SEN
15
Evaluate R
choose the smallest R
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:
In the above equation, I
been defined in
The peak reverse voltage for a boost converter is equal to the
regulator output voltage. The diode must be capable of han-
dling this peak reverse voltage. To improve efficiency, a low
forward drop Schottky diode is recommended.
POWER MOSFET SELECTION
The drive pin, DR, of the LM3481 must be connected to the
gate of an external MOSFET. In a boost topology, the drain
of the external N-Channel MOSFET is connected to the in-
ductor and the source is connected to the ground. The drive
pin voltage, V
performance characteristics). In most applications, a logic
level MOSFET can be used. For very low input voltages, a
sub-logic level MOSFET should be used.
The selected MOSFET directly controls the efficiency. The
critical parameters for selection of a MOSFET are:
1.
2.
3.
Minimum threshold voltage, V
On-resistance, R
Total gate charge, Q
SEN
DR
at the maximum and minimum V
Figure
, depends on the input voltage (see typical
I
D(Peak)
DS(ON)
SEN
13.
= [I
OUT
20136520
g
calculated.
OUT
is the output current and Δi
/ (1−D)] + Δi
TH(MIN)
L
IN
www.national.com
values and
L
has