MIC2155_0911 Datasheet, PDF (28/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
The snubber capacitor, C_S, is charged and discharged
each switching cycle. The energy stored in C_S is
dissipated by the snubber resistor, R_S, two times per
switching period. This power is calculated in the
equation below:
P_SNUBBER = f_S × C_S × V_IN2
Where:
f_S is the switching frequency for each phase
V_IN is the DC input voltage
Compensation of the Output Voltage Loop
The voltage regulation, filter and power stage section is
shown in Figure 24. The error amplifier for Channel 1 is
used to regulate the output voltage and compensate the
voltage regulation loop. It is a voltage output op amp that
is designed to use type III (PID) compensation. Type III
compensation has two compensating zeros, two poles
and a pole at the origin. The figure also shows the
transfer function for each section.
MIC2155/2156
Compensation is necessary to insure the control loop
has adequate bandwidth and phase margin to properly
respond to input voltage and output current transients.
High gain at DC and low frequencies is needed for
accurate output voltage regulation. Attenuation near the
switching frequency prevents switching frequency noise
from interfering with the control loop.
For this analysis, the LC filter in the two phase design is
combined into one. The inductances are in parallel and
the output capacitance is the total sum of all C_OUT. The
ESR is the parallel combination of all ESRs. The output
load is represented by a resistor R.
The output filter contains a complex double pole formed
by the capacitor and inductor and a zero from the output
capacitor and it’s ESR. The transfer function of the filter
is:
Where:
1+ ^s
Gfilter(s) =
1+
ωz
s
+ ⎜⎛ ^s ⎟⎞²
2 × Q × ωo _⎝ ωo _⎠
ωz =
1
C_O × Re sr
ωo =
1
C_O × L_O
Q = R × ^CO
L
C1
E/A
R2
C2
R3
C3
R1
R4
V_REF = 0.7V
Modulator
V_IN
Q1
Driver
Q2
Filter
L1 || L2
C_O
R
R_ESR
November 2009
Figure 24. Voltage Loop and Transfer Functions
28
M9999-111209-B