21DQ04 Ver la hoja de datos (PDF) - Monolithic Power Systems
Número de pieza
componentes Descripción
Fabricante
21DQ04 Datasheet PDF : 11 Pages
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MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
Compensation
The system stability is controlled through the
COMP pin. COMP 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. The DC loop gain is:
A VDC
=
VREF
VOUT
× A VEA
× GCS
× RLOAD
Where VREF is the feedback threshold voltage,
1.230V, AVEA is the transconductance error
amplifier voltage gain, 400 V/V, and GCS is the
current sense gain (roughly the output current
divided by the voltage at COMP), 3.5 A/V.
The system has 2 poles of importance; one is
due to the compensation capacitor (C4) and the
other is due to the output capacitor (C5). These
are:
fP1
=
(2π ×
GMEA
A VEA
×
C4)
Where fP1 is the first pole, and GMEA is the error
amplifier transconductance (770µS) and
fP2
=
1
(2π × RLOAD
× C5)
The system has one zero of importance due to
the compensation capacitor (C4) and the
compensation resistor (R3) which is
f Z1
=
1
(2π × R3 × C4)
If large value capacitors with relatively high
equivalent-series-resistance (ESR) are used,
the zero due to the capacitance and ESR of the
output capacitor can be compensated by a third
pole set by R3 and C3
fP3
=
1
(2π × R3 × C3)
The system crossover frequency fC, (the
frequency where the loop gain drops to 1, or
0dB) is important. A good rule of thumb is to set
the crossover frequency to approximately one
tenth of the switching frequency. In this case,
the switching frequency is 330KHz, so use a
crossover frequency of 33KHz. Lower
crossover frequencies result in slower response
and worse transient load recovery. Higher
crossover frequencies can result in instability.
Choosing the Compensation Components
The values of the compensation components
given in Table 4 yield a stable control loop for
the output voltage and given capacitor.
Table 4—Compensation Values for Typical
Output Voltage/Capacitor Combinations
VOUT
2.5V
C5
22µF Ceramic
3.3V 22µF Ceramic
5V 22µF Ceramic
12V 22µF Ceramic
2.5V 47µF SP-Cap
3.3V 47µF SP-Cap
5V
47µF SP-Cap
12V 47µF SP-Cap
2.5V
3.3V
5V
12V
560µF/6.3V, AL
30mΩ ESR
560µF/6.3V, AL
30mΩ ESR
470µF/10V, AL
30mΩ ESR
220µF/25V, AL
30mΩ ESR
Note: “AL” = Electrolytic
R3
3.9kΩ
5.1kΩ
7.5kΩ
18kΩ
8.2kΩ
10kΩ
16kΩ
36kΩ
C3
None
None
None
None
None
None
None
None
C4
4.7nF
3.9nF
2.7nF
1.2nF
2.2nF
2.2nF
1.5nF
1nF
100kΩ 150pF 1nF
120kΩ 120pF 1nF
150kΩ 82pF 1nF
180kΩ 33pF 1nF
MP1591 Rev. 2.3
www.MonolithicPower.com
7
9/27/2006
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© 2006 MPS. All Rights Reserved.
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