LTC3861-1 Datasheet, PDF (26/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Applications Information
Feedback Loop Compensation
The LTC3861-1 is a voltage mode controller with a second
dedicated current sharing loop to provide excellent phase-
to-phase current sharing in multiphase applications. The
current sharing loop is internally compensated.
While Type 2 compensation for the voltage control loop
may be adequate in some applications (such as with the
use of high ESR bulk capacitors), Type 3 compensation,
along with ceramic capacitors, is recommended for opti-
mum transient response. Referring to Figure 13, the error
amplifiers sense the output voltage at V_OUT.
The positive input of the error amplifier is connected to
an internal 600mV reference, while the negative input is
connected to the FB pin. The output is connected to COMP,
which is in turn connected to the line feedforward circuit
and from there to the PWM generator. To speed up the
overshoot recovery time, the maximum potential at the
COMP pin is internally clamped.
Unlike many regulators that use a transconductance (g_m)
amplifier, the LTC3861-1 is designed to use an inverting
summing amplifier topology with the FB pin configured
as a virtual ground. This allows the feedback gain to be
tightly controlled by external components, which is not
possible with a simple g_m amplifier. In addition, the voltage
feedback amplifier allows flexibility in choosing pole and
zero locations. In particular, it allows the use of Type 3
compensation, which provides a phase boost at the LC
pole frequency and significantly improves the control loop
phase margin.
In a typical LTC3861-1 circuit, the feedback loop consists
of the line feedforward circuit, the modulator, the external
inductor, the output capacitor and the feedback amplifier
with its compensation network. All these components
affect loop behavior and need to be accounted for in the
loop compensation. The modulator consists of the PWM
generator, the output MOSFET drivers and the external
MOSFETs themselves. The modulator gain varies linearly
with the input voltage. The line feedforward circuit com-
pensates for this change in gain, and provides a constant
gain from the error amplifier output to the inductor input
regardless of input voltage. From a feedback loop point of
view, the combination of the line feedforward circuit and
the modulator looks like a linear voltage transfer function
from COMP to the inductor input. It has fairly benign AC
behavior at typical loop compensation frequencies with
significant phase shift appearing at half the switching
frequency.
The external inductor/output capacitor combination
makes a more significant contribution to loop behavior.
These components cause a second order LC roll-off at the
output with 180° phase shift. This roll-off is what filters
the PWM waveform, resulting in the desired DC output
voltage, but this phase shift causes stability issues in the
feedback loop and must be frequency compensated. At
higher frequencies, the reactance of the output capacitor
will approach its ESR, and the roll-off due to the capacitor
will stop, leaving –20dB/decade and 90° of phase shift.
Figure 13 shows a Type 3 amplifier. The transfer function
of this amplifier is given by the following equation:
V_COMP
V_OUT
=
–₍1+ sC1R2_)[1+ s(R1+R3)C3_]
sR1(C1+ C2)⎡⎣1+ s(C1//C2)R2⎤⎦ ₍1+ sC3R3₎
26
V_OUT
R1
C2
C1
C3
R2
R3
–
FB
+
V_REF
–1
GAIN
0
COMP
PHASE
+1
–1
BOOST
FREQ
–90
–180
–270
–380
38611 F13
Figure 13. Type 3 Amplifier Compensation
38611f