Datasheet

V
CMP
0
t
VCC
t
CMP
t
CO
t
0.9V
PHASE (°)
FREQUENCY (Hz)
GAIN (dB)
80
60
40
20
0
-20
-40
-60
-80
90
45
0
-45
-90
-135
-180
-225
-270
1e-1 1e1 1e3 1e5 1e7
GAIN
60° Phase Margin
PHASE
ö
P2
ö
P3
ö
P1
ö
Z1
x=
0U
TT
-1
¸
¸
¹
·
¨
¨
©
§
s
Z
1
Z
xx +1
¸
¸
¹
·
¨
¨
©
§
s
Z
3P
+1
¸
¸
¹
·
¨
¨
©
§
s
Z
2P
+1
¸
¸
¹
·
¨
¨
©
§
s
Z
1P
1
3P
=Z
FSFS
CR x
1
2P
=Z
CMP
6
Ce5 x
:
LM3421, LM3421-Q1
LM3423, LM3423-Q1
SNVS574E –JULY 2008–REVISED MAY 2013 www.ti.com
(19)
It may also be necessary to add one final pole at least one decade above the crossover frequency to attenuate
switching noise and, in some cases, provide better gain margin. This pole can be placed across R
SNS
to filter the
ESL of the sense resistor at the same time. Figure 23 shows how the compensation is physically implemented in
the system.
The high frequency pole (ω
P3
) can be calculated:
(20)
The total system transfer function becomes:
(21)
The resulting compensated loop gain frequency response shown in Figure 24 indicates that the system has
adequate phase margin (above 45°) if the dominant compensation pole is placed low enough, ensuring stability:
Figure 24. Compensated Loop Gain Frequency Response
Figure 25. Start-Up Waveforms
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