0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

2

4

6

8

10

12

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

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:

(8)

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.

10

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