LM3481MMX National Semiconductor, LM3481MMX Datasheet - Page 15

• Manufacturer Part Number: LM3481MMX
• Description: High Efficiency Low-Side N-Channel Controller for Switching Regulators
• Manufacturer: National Semiconductor

PROGRAMMING THE OUTPUT VOLTAGE AND OUTPUT
CURRENT
The output voltage can be programmed using a resistor di-
vider between the output and the feedback pins, as shown in
Figure 14. The resistors are selected such that the voltage at
the feedback pin is 1.275V. R
the equation,
A 100 pF capacitor may be connected between the feedback
and ground pins to reduce noise.
The maximum amount of current that can be delivered at the
output can be controlled by the sense resistor, R
limit occurs when the voltage that is generated across the
sense resistor equals the current sense threshold voltage,
V
characteristics section. This can be expressed as:
CURRENT LIMIT WITH ADDITIONAL SLOPE
COMPENSATION
If an external slope compensation resistor is used (see Figure
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
SENSE
SL
is used, then this will add to the existing slope compen-
. Limits for V
SENSE
is a defined parameter in the electrical char-
SENSE
V
CS
= V
have been specified in the electrical
SL
SENSE
F1
is the additional slope compen-
and R
CS
− ΔV
, will then be given by:
F2
SL
can be selected using
FIGURE 14. Adjusting the Output Voltage
SEN
SEN
to:
. Current
15
The peak current through the switch is equal to the peak in-
ductor current.
Therefore for a boost converter
Combining the two equations yields an expression for R
Evaluate R
choose the smallest 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,
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 Figure 13.
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.
SEN
SEN
at the maximum and minimum V
pin quiet.
I
D(Peak)
I
sw(peak)
SL
I
20136520
sw(peak)
= R
SEN
SL
= [I
OUT
SL
x R
can also be used to help filter noise
calculated.
= I
OUT
x K as defined earlier, R
is the output current and Δi
SEN
L
(max) + Δi
/ (1−D)] + Δi
= V
SENSE
L
L
IN
www.national.com
values and
SL
can be
L
SEN
has