MP2309 Просмотр технического описания (PDF) - Monolithic Power Systems
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MP2309 Datasheet PDF : 10 Pages
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MP2309 – 1A, 23V, 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:
fZ1
=
1
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:
fESR
=
1
2π × C2 × RESR
In this case (as shown in Figure 2), a third pole
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:
fP3
=
1
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. 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 below one-tenth of the
switching frequency. Determine the R3 value by
the following equation:
R3 = 2π × C2 × fC × VOUT < 2π × C2 × 0.1× fS × VOUT
GEA × GCS
VFB
GEA × GCS
VFB
Where fC is the desired crossover frequency
which is typically 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.
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.
Determine the C3 value by the following equation:
C3 >
4
2π × R3 × fC
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:
1
< fS
2π × C2 × RESR 2
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:
C6 = C2 × RESR
R3
External Bootstrap Diode
It is recommended that an external bootstrap
diode be added when the system has a 5V
fixed input or the power supply generates a 5V
output. This helps improve the efficiency of the
regulator. The bootstrap diode can be a low
cost one such as IN4148 or BAT54.
5V
BS
MP2309
SW
10nF
MP2309_F02
Figure 2—External Bootstrap Diode
This diode is also recommended for high duty
cycle operation (when VOUT >65%) and high
VIN
output voltage (VOUT>12V) applications.
MP2309 Rev. 1.4
www.MonolithicPower.com
8
3/13/2006
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