Datasheet
-60
-40
60
10
1
10
2
10
3
10
4
10
5
Frequency (Hz)
-20
0
20
40
10
6
0
-50
-100
-150
-180
-250
-200
PHASE ( )
MAGNITUDE (dB)
BW = 16.7 kHz
PM = 61 degrees
C
C2
=
50 x 10
3
+ 900
2S (.159 x 10
3
) (50 x 10
3
) (900)
= 1.1 nF
1
2SC
OUT
R
ESR
250kHz
<
2S(20 x 10
3
)(900)
3.16
1
d
d
C
C1
d
d
C
C1
28 nF 62 nF
2S(2.86 x 10
3
)(900)
R
C
=
= 904:
(15.5)(0.001)(50 x 10
3
)(0.508)(2.86 x 10
3
) - (20 x 10
3
)
(20 x 10
3
)(50 x 10
3
)
LM3477
SNVS141K –OCTOBER 2000–REVISED MARCH 2013
www.ti.com
(57)
This sets the high frequency gain of the compensator such that a crossover frequency of f
C
is obtained. The
capacitor C
C1
sets the compensator zero, f
Z2
. Set f
Z2
between the power pole f
P1
and the ½ decade before the
target crossover frequency f
C
:
(58)
Choosing C
C1
= 62 x 10
−9
F will set f
Z2
= f
P1
, canceling out the power pole and insuring a −20dB/decade slope in
the low frequency magnitude response. In other words, the phase margin below the crossover frequency will
always be higher than the phase margin at the crossover frequency.
If better transient response times are desired, a second method is to set f
Z2
between f
P1
and ½ decade before f
C
,
the target crossover frequency. This trades more low frequency gain for less phase margin, which translates to
faster but more oscillatory step responses. We pick C
C1
= 47nF.
If the esr zero of the output capacitor (f
ESR
) is too low or if more phase margin is required, additional components
may be added to increase the flexibility of the compensator.
Use C
C2
if f
ESR
< ½ f
S
, that is if:
(59)
For this example, f
ESR
= 159 kHz, so use C
C2
.
(60)
The equations used here for R
C
, C
C1
, and C
C2
are approximations valid when C
C2
<< C
C1
. For exact equations,
see PLOTTING THE OPEN LOOP RESPONSE. In some cases, the desired inductance is several times higher
than the optimal inductance set by the internal slope compensation. This results in a Q lower than 0.15, in which
case additional methods of compensating are presented (see SAMPLING POLE QUALITY FACTOR).
Figure 34. Open Loop Frequency Response for LM3477 Compensation Design Example
26 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated
Product Folder Links: LM3477