Datasheet

LTC3112
20
3112fc
For more information www.linear.com/LTC3112
For charging, LED lighting, or other applications that do not
require an optimized output voltage transient re-sponse, a
simple Type I compensation network as shown in Figure
5 can be used to stabilize the voltage loop. To ensure suf
-
ficient phase margin, the gain of the error am-plifier must
be
low enough that the resultant crossover frequency of
the control loop is well below the resonant frequency.
+
–
C1
GND
LTC3112
V
COMP
3112 F05
FB
V
OUT
R
BOT
R
TOP
0.8V
In most applications, the low bandwidth of the Type I com-
pensated loop wi
ll not provide sufficient transient response
performance. To obtain a wider bandwidth feedback loop,
optimize the transient response, and minimize the size of
the output capacitor, a Type III com-pensation network
as shown in Figure 6 is required.
Figure 5. Error Amplifier with Type I Compensation
C
FB
R
FB
GND
LTC3112
V
COMP
3112 F06
FB
V
OUT
R
BOT
R
TOP
R
FF
C
FF
0.8V
C
POLE
+
–
Figure 6. Error Amplifier with Type III Compensation
A Bode plot of the typical Type III compensation network
is shown in Figure 7. The Type III compensation network
provides a pole near the origin which produces a very high
loop gain at DC to minimize any steady state error in the
regulation voltage. Tw o zeros located at f
ZERO1
and f
ZERO2
provide sufficient phase boost to allow the loop crossover
frequency to be set above the resonant frequency, f
O
, of
the power stage. The Type III compensation network also
introduces a second and third pole. The second pole, at
frequency f
POLE2
, reduces the error amplifier gain to a
zero slope to prevent the loop crossover from extending
too high in frequency. The third pole at frequency f
POLE3
provides attenuation of high frequency switching noise.
applicaTions inForMaTion
f
ZERO1
PHASE
90°
–90°
0°
GAIN
–20dB/DEC
–20dB/DEC
f
ZERO2
3112 F07
f
f
POLE2
f
POLE3
Figure 7. Type III Compensation Bode Plot.
The transfer function of the compensated Type III error
amplifier from the input of the resistor divider to the output
of the error amplifier, V
COMP
, is:
V
COMP
(s)
V
OUT
(s)
= G
EA
1+
s
2πf
ZERO1






1+
s
2πf
ZERO2






s 1+
s
2πf
POLE1






1+
s
2πf
POLE2






The error amplifier gain is given by the following equation.
The simpler approximate value is sufficiently accurate in
most cases since C
FB
is typically much larger in value
than C
POLE
.
G
EA
=
1
R
TOP
C
FB
+ C
POLE
( )
≅
1
R
TOP
C
FB
The pole and zero frequencies of the Type III compensation
network can be calculated from the following equations
where all frequencies are in Hz, resistances are in ohms,
and capacitances are in farads.