MP1482 – 2A, 18V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
In this case (as shown in Figure 2), a third pole
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:
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:
fS
2
1
1
fP3
=
<
2π× C6×R3
2π × C2× RESR
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 rule of thumb is to set
the crossover frequency below one-tenth of the
switching frequency.
If this is the case, then add the second
compensation capacitor (C6) to set the pole fP3
at the location of the ESR zero. Determine the
C6 value by the equation:
C2 × RESR
C6 =
R3
External Bootstrap Diode
An external bootstrap diode may enhance the
efficiency of the regulator, and it will be a must
if the applicable condition is:
To optimize the compensation components, the
following procedure can be used.
z VOUT is 5V or 3.3V, and duty cycle is high:
VOUT
1. Choose the compensation resistor (R3) to set
the desired crossover frequency.
D=
>65%
VIN
Determine the R3 value by the following
equation:
In these cases, an external BST diode is
recommended from the output of the voltage
regulator to BST pin, as shown in Figure.2
2π × C2 × fC VOUT 2π × C2 × 0.1× fS VOUT
R3 =
×
<
×
GEA × GCS
VFB
GEA × GCS
VFB
External BST Diode
IN4148
Where fC is the desired crossover frequency
which is typically below one tenth of the
switching frequency.
BST
CBST
MP1482
5V or 3.3V
SW
+
COUT
L
2. Choose the compensation capacitor (C3) to
achieve the desired phase margin. For
applications with typical inductor values, setting
the compensation zero, fZ1, below one-forth of
the crossover frequency provides sufficient
phase margin.
Figure 2—Add Optional External Bootstrap
Diode to Enhance Efficiency
The recommended external BST diode is
IN4148, and the BST cap is 0.1~1µF.
Determine the C3 value by the following equation:
4
C3 >
2π × R3 × fC
Where R3 is the compensation resistor.
MP1482 Rev. 1.3
9/21/2011
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