LM2698 datasheet(13/17 Pages) NSC

13 / 17 page

Operation (Continued)

Additional Comments on the Open Loop Frequency Re-

sponse

The procedure used here to pick the compensation network

will provide a good starting point. In most cases, these

values will be sufficient for a stable design. It is always

recommended to check the design in a real test setup. This

is easy to do with the aid of a dynamic load. Set the high and

low load values to your system requirements and switch

between the two at about 1kHz. View the output voltage with

an oscilloscope using AC coupling, and zoom in enough to

see the waveform react to the load change. Use the follow-

ing table to determine if your design is stable. Remember to

use worst case conditions (V

IN(MIN),ROUT(MIN),ROUT(MAX)).

Response

Conclusion

What to

Change

Underdamped,

weak attenuation

Nearing instability

Make C

larger

Underdamped,

strong attenuation

Stable

Nothing

Critically damped

Stable

Nothing

Overdamped

Stable

Nothing

Application Information

1.25MHz Boost Converter

Figure 6 shows the LM2698 boosting 3.3V to 10V at 300mA.

As discussed in the COMPENSATION section, the R

DS(ON)

of the internal FET in the LM2698 raises as the input voltage

drops below 5V (see Typical Performance Characteristics).

The minimum input voltage for this application is 2.5V, at

which point the R

DS(ON) is approximately 200mΩ. Substitut-

ing these values in for

Equation (1), it is found that either a

10 µH (1.25MHz operation) or a 22 µH (600kHz operation) is

necessary for a stable design. The circuit is operated at

1.25MHz to allow for a smaller inductance. From the Com-

pensator Design equations, R

C is calculated to be 18.6kΩ,

and a 20k

Ω resistor is used.

20012668

FIGURE 6. 3.3V to 10V Boost Converter

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