If V

in the inductor changes at a constant rate. This is the case in

DC/DC converters since the voltages at the input and output

can be approximated as a constant. The current through the

inductor of the LM2698 boost converter is shown in

Figure

2(a). The important quantities in determining a proper induc-

tance value are I

(the inductor current ripple). If

∆i

inductor current will drop to zero for a portion of the cycle and

the converter will operate in discontinuous conduction mode.

If

∆i

above zero and the converter will operate in continuous

conduction mode (CCM). All the analysis in this datasheet

assumes operation in continuous conduction mode. To op-

erate in CCM:

I

Choose the minimum I

CCM operation. A common choice is to set

∆i

I

The inductance value will also affect the stability of the

converter. Because the LM2698 utilizes current mode con-

trol, the inductor value must be carefully chosen. See the

COMPENSATION section for recommended inductance val-

ues.

Choosing an appropriate core size for the inductor involves

calculating the average and peak currents expected through

the inductor. In a boost converter,

and

I

where

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

Where I

teristics table and Typical Performance Curves). Hence, as

V

the load increases, as shown in

Figure 3.

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 capaci-

tor. In the second cycle, the transistor is open and the diode

is forward biased; the load current is supplied by the induc-

tor.

Observation of the boost converter circuit shows that the

average current through the diode is the average load cur-

rent, 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.

Input Capacitor

Due to the presence of an inductor at the input of a boost

converter, the input current waveform is continuous and

triangular. The inductor ensures that the input capacitor sees

fairly low ripple currents. However, as the inductor gets

smaller, the input ripple increases. The rms current in the

input capacitor is given by:

The input capacitor should be capable of handling the rms

current. Although the input capacitor is not so critical in boost

applications, a 10 µF or higher value, good quality capacitor

prevents any impedance interactions with the input supply.

A 0.1µF or 1µF ceramic bypass capacitor is also recom-

mended on the V

20012673

FIGURE 3. Maximum Output Current vs Input Voltage

www.national.com

9