Datasheet
V
OUT
= 1.26
(
1+
R
F1
R
F2
(
2f
S
L
'i
L =
DV
IN
LM3488
LM3488-Q1
SNVS089M –JULY 2000–REVISED MARCH 2013
www.ti.com
The important quantities in determining a proper inductance value are I
L
(the average inductor current) and Δi
L
(the inductor current ripple). If Δi
L
is larger than I
L
, the inductor current will drop to zero for a portion of the cycle
and the converter will operate in discontinuous conduction mode. If Δi
L
is smaller than I
L
, the inductor current will
stay above 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 conduction mode, the following
conditions must be met:
I
L
> Δi
L
(9)
(10)
(11)
Choose the minimum I
OUT
to determine the minimum L. A common choice is to set Δi
L
to 30% of I
L
. Choosing an
appropriate core size for the inductor involves calculating the average and peak currents expected through the
inductor. In a boost converter,
(12)
and I
L_peak
= I
L
(max) + Δi
L
(max),
where
(13)
A core size with ratings higher than these values should be chosen. If the core is not properly rated, saturation
will dramatically reduce overall efficiency.
The LM3488 can be set to switch at very high frequencies. When the switching frequency is high, the converter
can be operated 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 LM3488 senses the peak current through the switch. The peak current through the switch is the same as the
peak current calculated above.
PROGRAMMING THE OUTPUT VOLTAGE
The output voltage can be programmed using a resistor divider between the output and the feedback pins, as
shown in Figure 34. The resistors are selected such that the voltage at the feedback pin is 1.26V. R
F1
and R
F2
can be selected using the equation,
(14)
A 100pF capacitor may be connected between the feedback and ground pins to reduce noise.
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