LTC3115-2_15 Datasheet, PDF (24/42 Pages) Linear Technology – 40V, 2A Synchronous Buck-Boost DC/DC Converter

LTC3115-2
Applications Information
Compensation of the Voltage Loop
The small-signal models of the LTC3115-2 reveal that the
transfer function from the error amplifier output, VC, to
the output voltage is characterized by a set of resonant
poles and a possible zero generated by the ESR of the
output capacitor as shown in the Bode plot of Figure 7.
In boost mode operation, there is an additional right half
plane zero that produces phase lag and increasing gain at
higher frequencies. Typically, the compensation network
is designed to ensure that the loop crossover frequency
is low enough that the phase loss from the right half
plane zero is minimized. The low frequency gain in buck
mode is a constant, but varies with both V_IN and V_OUT in
boost mode.
V_OUT
R_TOP
R_BOT
LTC3115-2
1000mV +
FB
–
C1
VC
GND
31152 F08
Figure 8. 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 compensation network as
shown in Figure 9 is required.
GAIN
0°
–90°
–180°
–270°
PHASE
–40dB/DEC
–20dB/DEC
BUCK MODE
BOOST MODE
V_OUT
R_TOP
R_BOT
R_FF
C_FF
C_{FB RFB}
C_POLE
LTC3115-2
1000mV +
FB
–
VC
GND
31152 F09
Figure 9. Error Amplifier with Type III Compensation
f
f_O
f_RHPZ
31152 F07
Figure 7. Buck-Boost Converter Bode Plot
For charging or other applications that do not require an
optimized output voltage transient response, a simple Type
I compensation network as shown in Figure 8 can be used
to stabilize the voltage loop. To ensure sufficient phase
margin, the gain of the error amplifier must be low enough
that the resultant crossover frequency of the control loop
is well below the resonant frequency.
A Bode plot of the typical Type III compensation network
is shown in Figure 10. 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
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.
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