Datasheet
10e1 10e2 10e3 10e4 10e5 10e6
-40
-20
0
20
40
60
O
f
RHP
O
f
Z2
X
f
P1
X
f
P
2
X
f
n
X
DSON
Frequency (Hz)
Magnitude(dB)
10e1
10e2
10e3 10e4 10e5 10e6
-200
-150
-100
-50
0
O
O
X
X
Frequency (Hz)
Phase (
q
)
Powerstage
Compensator
T
Bandwidth = 6.9kHz, Phase Margin = 78.4 , R = 0.15
:
, L = 10uH
Duty Cycle (1-V
IN
/V
OUT
)
Q = 0.5
1
25
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Inductance (PH)
Q = 5
(b)
V
OUT
= 12V
5
10
15
20
1
50
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Duty Cycle (1-V
IN
/V
OUT
)
Inductance (PH)
Q = 0.5
Q = 5
(a)
V
OUT
= 12V
45
40
35
30
25
20
15
10
5
LM2698
SNVS153E –MAY 2001–REVISED APRIL 2013
www.ti.com
Figure 17. Minimum Inductance Requirements for Figure 18. Minimum Inductance Requirements for
(a) f
S
= 600kHz (b) f
S
= 1.25MHz
The goal of the compensation network is to provide the best static and dynamic performance while insuring
stability over line and load variations. The relationship of stability and performance can be best analyzed by
plotting the magnitude and phase of the open loop frequency response in the form of a bode plot. A typical bode
plot of the LM2698 open loop frequency response is shown in Figure 19.
Figure 19. Bode plot of the LM2698 Frequency Response using the Typical Application Circuit
Poles are marked with an 'X', and zeros are marked with a 'O'. The bolded 'O' labeled 'f
RHP
' is a right-half plane
zero. Right half plane zeros act like normal zeros to the magnitude (+20dB/decade slope influence) and like
poles to the phase (−90° shift). Three curves are shown. The powerstage curve is the frequency response of the
powerstage, which includes the switch, diode, inductor, output capacitor, and load. The compensator curve is the
frequency response of the compensator, which is the error amp combined with the compensation network. T is
the product of the powerstage and the compensator and is the complete open loop frequency response. The
power stage response is fixed by line and load constraints, while the compensator is set by the external
compensation network at pin 1. The compensator can be designed in a few simple steps as follows.
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