LTC3112_15 Datasheet, PDF (20/32 Pages) Linear Technology – 15V, 2.5A Synchronous Buck-Boost DC/DC Converter

LTC3112
Applications Information
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.
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.
V_OUT
LTC3112
R_TOP
0.8V +
FB
–
R_BOT ^C1 _VCOMP
GND
3112 F05
Figure 5. Error Amplifier with Type I Compensation
In most applications, the low bandwidth of the Type I com-
pensated loop will 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.
V_OUT
R_TOP
R_BOT
R_FF
C_FF
C_{FB RFB}
C_POLE
LTC3112
0.8V +
FB
–
V_COMP
GND
3112 F06
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. Two zeros located at f_ZERO1 and f_ZERO2
provide sufficient phase boost to allow the loop crossover
GAIN
–20dB/DEC
–20dB/DEC
90°
0°
–90°
PHASE
f_ZERO1
f_POLE2 f_POLE3
f_ZERO2
f
3112 F07
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+

2π
s
f_ZERO2



s

1+

2π
f_P^s_OLE1^{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
=
R_TOP
1
(^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.
3112fc
20
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