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M2305SDS-Z View Datasheet(PDF) - Monolithic Power Systems

Part Name
Description
Manufacturer
M2305SDS-Z
MPS
Monolithic Power Systems MPS
M2305SDS-Z Datasheet PDF : 12 Pages
1 2 3 4 5 6 7 8 9 10 Next Last
MP2305S – 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
The DC gain of the voltage feedback loop is
given by:
A VDC
= RLOAD × GCS
× A EA
×
VFB
VOUT
Where AVEA is the error amplifier voltage gain;
GCS is the current sense transconductance 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 the error amplifier, and the
other is due to the output capacitor and the load
resistor. These poles are located at:
fP1
=
GEA
2π × C3 × A VEA
fP2
=
1
2π × C2 × RLOAD
Where GEA is the error amplifier transconductance.
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, 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
3. Determine if the second compensation
capacitor (C6) is required. It is required if the
crossover frequencies could cause system
instability. A good rule of thumb is to set the
crossover frequency below one-tenth of the
switching frequency.
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.
Table 3—Compensation Values for Typical
Output Voltage/Capacitor Combinations
VOUT
1.8V
L1
6.8uH
3.3V 10μH
5.0V 15μH
12.0V 22μH
C2
22μF/6.3V
Ceramic
22μF/6.3V
Ceramic
22μF/6.3V
Ceramic
22μF/16V
Ceramic
R3 C3 C6
3.3k5.6nF None
5.6k3.3nF None
10k2.2nF None
15k1.0nF None
To optimize the compensation components, the
following procedure can be used.
1. Choose the compensation resistor (R3) to set
the desired crossover 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.
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.
ESR zero of the output capacitor is located at
MP2305S Rev. 1.02
www.MonolithicPower.com
9
10/22/2013
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
 

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