Datasheet

V
o
V
c
R
2
R
1
g
m
compensation
network
C
C1
C
C2
R
C1
R
C2
C
c1
=
1
2S 297 Hz 9.76k
x
x
|56 nF
R
c1
=
20k + 59k
20k
|9.76k
1350P
3.3
x
LM2717-ADJ
www.ti.com SNVS407C –DECEMBER 2005–REVISED MARCH 2013
where
• B is the desired gain in V/V at fp (fz1)
• gm is the transconductance of the error amplifier
• 1 and R2 are the feedback resistors as shown in Figure 19 (13)
A gain value around 10dB (3.3v/v) is generally a good starting point.
Example: B = 3.3 v/v, gm=1350µmho, R1 = 20 KΩ, R2 = 59 KΩ:
(14)
Bandwidth will vary proportional to the value of Rc1. Next, Cc1 can be determined with the following equation:
(15)
Example: fpmin = 297 Hz, Rc1 = 20 KΩ:
(16)
The value of C
c1
should be within the range determined by fpmin/max. A higher value will generally provide a
more stable loop, but too high a value will slow the transient response time.
The compensation network (Figure 19) will also introduce a low frequency pole which will be close to 0Hz.
A second pole should also be placed at fz. This pole can be created with a single capacitor Cc2 and a shorted
Rc2 (see Figure 19). The minimum value for this capacitor can be calculated by:
(17)
Cc2 may not be necessary, however it does create a more stable control loop. This is especially important with
high load currents.
Example: fz = 80 kHz, Rc1 = 20 KΩ:
(18)
A second zero can also be added with a resistor in series with Cc2. If used, this zero should be placed at fn,
where the control to output gain rolls off at -40dB/dec. Generally, fn will be well below the 0dB level and thus will
have little effect on stability. Rc2 can be calculated with the following equation:
(19)
Figure 19. Compensation Network
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