Datasheet

0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
2 4
6
8
10
12
V
IN
(V)
I
OUT
(A)
V
OUT
= 12V
'i
L
=
DV
IN
2Lf
s
I
L(AVG)
=
I
OUT(AVG)
1-D
LM2698
SNVS153E –MAY 2001–REVISED APRIL 2013 www.ti.com
The inductance value will also affect the stability of the converter. Because the LM2698 utilizes current mode
control, the inductor value must be carefully chosen. See the Compensation section for recommended
inductance values.
Choosing an appropriate core size for the inductor involves calculating the average and peak currents expected
through the inductor. In a boost converter,
(6)
and
I
L(Peak)
= I
L(AVG)
+ Δi
L
,
where
(7)
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.
Current Limit
The current limit in the LM2698 is referenced to the peak switch current. The peak currents in the switch of a
boost converter will always be higher than the average current supplied to the load. To determine the maximum
average output current that the LM2698 can supply, use:
I
OUT(MAX)
= (I
CL
− Δi
L
)*(1−D) = (I
CL
− Δi
L
)*V
IN
/V
OUT
(8)
Where I
CL
is the switch current limit (see Electrical Charateristics table and Typical Performance Curves). Hence,
as V
IN
increases, the maximum current that can be supplied to the load increases, as shown in Figure 16.
Figure 16. Maximum Output Current vs Input Voltage
Diode
The diode in a boost converter such as the LM2698 acts as a switch to the output. During the first cycle, when
the transistor is closed, the diode is reverse biased and current is blocked; the load current is supplied by the
output capacitor. In the second cycle, the transistor is open and the diode is forward biased; the load current is
supplied by the inductor.
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 its peak current. To improve efficiency, a low forward drop Schottky diode is
recommended.
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