Datasheet

Battery or Power
Source
2.5V to 3.3V
L1
10PH
D
C
IN
22PF
R
C
20k
C
C
4.7nF
R
FB2
140k
R
FB1
1M
C
OUT
10PF
1
4
3
5
6 7
2
FSLCT
FB
GND
V
C
SHDN
V
IN
SW
LM2698
10V
LM2698
SNVS153E –MAY 2001–REVISED APRIL 2013 www.ti.com
Additional Comments on the Open Loop Frequency Response
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 following table to
determine if your design is stable. Remember to use worst case conditions (V
IN(MIN)
, R
OUT(MIN)
, R
OUT(MAX)
).
Response Conclusion What to Change
Underdamped, weak attenuation Nearing instability Make C
C1
larger
Underdamped, strong attenuation Stable Nothing
Critically damped Stable Nothing
Overdamped Stable Nothing
APPLICATION INFORMATION
Figure 20. 3.3V to 10V Boost Converter
1.25MHz Boost Converter
Figure 20 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. Substituting these values in for Equation 13, 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 Compensator Design equations, R
C
is calculated to be 18.6kΩ, and a
20kΩ resistor is used.
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