MP18561 – 3A, 28V, 385KHz STEP-DOWN CONVERTER
Compensation Components
In this case (as shown in Figure 3), a third pole
MP18561 employs current mode control for
easy compensation and fast transient response.
The system stability and transient response are
controlled through the COMP pin. COMP pin is
the output of the internal transconductance
error amplifier. A series capacitor-resistor
combination sets a pole-zero combination to
control the characteristics of the control system.
set by the compensation capacitor (C6) and the
compensation resistor (R3) is used to
compensate the effect of the ESR zero on the
loop gain. This pole is located at:
1
fP3
=
2π × C6 × R3
The goal of compensation design is to shape
the converter transfer function to get a desired
loop gain. The system crossover frequency
where the feedback loop has the unity gain is
important.
The DC gain of the voltage feedback loop is
given by:
VFB
AVDC = RLOAD × GCS × AVEA
×
VOUT
Lower crossover frequencies result in slower
line and load transient responses, while higher
crossover frequencies could cause system
unstable. A good rule of thumb is to set the
crossover frequency to approximately one-tenth
of the switching frequency. Switching frequency
for the MP18561 is 385KHz, so the desired
crossover frequency is around 38KHz.
Where AVEA is the error amplifier voltage gain,
400V/V, GCS is the current sense
transconductance, 5.9A/V, and RLOAD is the load
resistor value.
The system has two poles of importance. One
is due to the compensation capacitor (C3) and
the output resistor of error amplifier, and the
other is due to the output capacitor and the load
resistor. These poles are located at:
Table 3 lists the typical values of compensation
components for some standard output voltages
with various output capacitors and inductors.
The values of the compensation components
have been optimized for fast transient
responses and good stability at given conditions.
GEA
fP1
=
2π× C3× AVEA
1
fP2
=
2π × C2× RLOAD
Where
GEA
is
the
error
amplifier
transconductance, 800μA/V.
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). This zero is located
at:
1
fZ1
=
2π × C3×R3
The system may have another zero of
importance, if the output capacitor has a large
capacitance and/or a high ESR value. The zero,
due to the ESR and capacitance of the output
capacitor, is located at:
1
fESR
=
2π × C2× RESR
MP18561 Rev. 0.1
12/12/2007
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