MIC2155_0911 Datasheet, PDF (31/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
Step 6: Determine the frequency for fp2
This is the high frequency pole, which is useful in
additional attenuation of the switching frequency. It
should initially be set at half of the switching frequency. If
it is set too low, it will lower the phase margin at the
crossover frequency, making it difficult to achieve the
proper phase margin. If set too high, it will not provide
attenuation of the switching frequency, which could lead
to jitter of the switching waveform or instability under
certain conditions.
fp2 = ^fs
2
Calculating Error Amplifier Component Values
Once the pole and zero frequencies have been fixed, the
error amplifier’s resistor and capacitor values are
calculated.
R1: This value is chosen first. All other component
values are calculated from R1. A value of 10K is
suggested. If R1 is chosen too high, R2 may be very
large and the high impedances could be sensitive to
noise. If the remote sense amplifier is used, R1 must be
large enough so than not more than 500µA of current is
drawn from the amplifier.
R2: The value of R2 is determined from the mid-band
gain of the error amplifier. This gain depends on the
frequencies of the poles, zeros and LC filter resonant
frequency.
MIC2155/2156
Based on the amount of gain necessary at the crossover
frequency the mid-band gain and R2 value is calculated
using the following formula:
G_CO
=
Vm _{× ⎜⎛}
H× Vin _⎝
fo _⎟⎞²
fc _⎠
× ⎜⎛
⎝
fc _{⎟⎞ ×}
fz _⎠
fz2
fp1
R2 = R1× G_CO
The other component values are calculated as follows:
C2 =
1
2 × π × fz1× R2
C3 =
1
2 × π × fz2 × R1
C3 =
C2
2 × π × (fp1× C2 × R2 − 1)
R3 =
1
2 × π × fp2 × C3
Compensation of the Current Sharing Loop
The control circuitry for Channel 2 forces the channel’s
output current to match the current in Channel 1. The
Channel 2 error amplifier compares the inductor currents
in the two channels and adjusts the duty cycle of
Channel 2 to control its output current. A block diagram
is shown in Figure 27.
Channel 2
Transconductance E/A
RZ1
CZ1
Ramp
Modulator
V_IN
Q1
Driver
Q2
Filter
R2
C2
R_L2
L2
November 2009
Channel 1 I_L
R_L1
R1
L1
C1
Figure 27. Current Sharing Loop and Transfer Functions
31
C_O
M9999-111209-B