MP1484 – 3A, 18V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER
The system has one zero of importance, due to the
compensation capacitor (C3) and the compensation
resistor (R3). This zero is located at:
f
Z1
=
1
2
π ×
C3
×
R3
2. Choose the compensation capacitor (C3) to
achieve the desired phase margin. For
applications with typical inductor values, setting
the compensation zero (f
Z1
) below one-forth of
the crossover frequency provides sufficient
phase margin.
Determine C3 by the following equation:
C3
>
4
2
π ×
R3
×
f
C
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:
f
ESR
=
1
2
π ×
C2
×
R
ESR
Where R3 is the compensation resistor.
3. Determine if the second compensation
capacitor (C6) is required. It is required if the
ESR zero of the output capacitor is located at
less than half of the switching frequency, or the
following relationship is valid:
f
1
<
S
2
π ×
C2
×
R
ESR
2
In this case, a third pole set by
compensation capacitor (C6) and
compensation resistor (R3) is used
compensate the effect of the ESR zero on
loop gain. This pole is located at:
f
P 3
=
1
2
π ×
C6
×
R3
the
the
to
the
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. Lower crossover frequencies result
in slower line and load transient responses,
while higher crossover frequencies could cause
system instability. A good standard is to set the
crossover frequency below one-tenth of the
switching frequency.
To optimize the compensation components, the
following procedure can be used.
1. Choose the compensation resistor (R3) to set
the desired crossover frequency.
Determine R3 by the following equation:
2
π ×
C2
×
f
C
V
OUT
2
π ×
C2
×
0.1
×
f
S
V
OUT
×
<
R3
=
×
G
EA
×
G
CS
V
FB
G
EA
×
G
CS
V
FB
If this is the case, then add the second
compensation capacitor (C6) to set the pole f
P3
at the location of the ESR zero. Determine C6
by the equation:
C6
=
C2
×
R
ESR
R3
External Bootstrap Diode
An external bootstrap diode may enhance the
efficiency of the regulator, the applicable
conditions of external BS diode are:
V
OUT
is 5V or 3.3V; and
Duty cycle is high: D=
V
OUT
>65%
V
IN
In these cases, an external BS diode is
recommended from the output of the voltage
regulator to BS pin, as shown in Fig.2
External BST Diode
IN4148
BS
C
BST
5V or 3.3V
C
OUT
Where f
C
is the desired crossover frequency
which is typically below one tenth of the
switching frequency.
MP1484
SW
L
Figure 2—Add Optional External Bootstrap
Diode to Enhance Efficiency
The recommended external BS diode is IN4148,
and the BS cap is 0.1~1µF.
MP1484 Rev. 0.9
10/20/2008
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