LM3481EVAL/NOPB National Semiconductor, LM3481EVAL/NOPB Datasheet - Page 14

• 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

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POWER INDUCTOR SELECTION
The inductor is one of the two energy storage elements in a
boost converter.
varies during a switching cycle. The current through an in-
ductor is quantified as:
FIGURE 13. a. Inductor current b. Diode current c. Switch
If V
given input voltage and output voltage, the current in the in-
ductor changes at a constant rate.
The important quantities in determining a proper inductance
value are I
tor current ripple difference between the peak inductor current
and the average inductor current). If Δi
inductor current will drop to zero for a portion of the cycle and
the converter will operate in discontinuous conduction mode.
If Δi
zero and the converter will operate in continuous conduction
mode. All the analysis in this datasheet assumes operation in
continuous conduction mode. To operate in continuous con-
duction mode, the following conditions must be met:
L
L
(t) is constant, di
is smaller than I
L
(the average inductor current) and Δi
Figure 13
L
L
(t)/dt must be constant. Hence, for a
, the inductor current will stay above
current
I
L
shows how the inductor current
> Δi
L
L
is larger than I
L
(the induc-
20136524
L
, the
14
Choose the minimum I
common choice is to set (2 x Δi
appropriate core size for the inductor involves calculating the
average and peak currents expected through the inductor. In
a boost converter,
A core size with ratings higher than these values should be
chosen. If the core is not properly rated, saturation will dra-
matically reduce overall efficiency.
The LM3481 can be set to switch at very high frequencies.
When the switching frequency is high, the converter can op-
erate with very small inductor values. With a small inductor
value, the peak inductor current can be extremely higher than
the output currents, especially under light load conditions.
The LM3481 senses the peak current through the switch. The
peak current through the switch is the same as the peak cur-
rent calculated above.
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
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:
The peak current through the switch is equal to the peak in-
ductor current.
Therefore for a boost converter
SENSE
14. The resistors are selected such that the voltage at
. Limits for V
I
L_peak
I
sw(peak)
I
SENSE
sw(peak)
= I
OUT
have been specified in the electrical
L
x R
(max) + Δi
= I
to determine the minimum L. A
F1
SEN
L
(max) + Δi
and R
L
) to 30% of I
= V
F2
SENSE
L
(max)
can be selected using
L
L
. Choosing an
SEN
. Current