MP1583 MPS, MP1583 Datasheet - Page 7

• Manufacturer Part Number: MP1583
• Description: Step Down Converter
• Manufacturer: MPS

The system has one zero of importance, due
to the compensation capacitor (C3) and the
compensation resistor (R3). The zero is:
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 C6. The pole is:
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
1/5 of the switching frequency. In this case,
the switching frequency is 385KHz, so use a
crossover frequency, f
crossover
response and worse transient load recovery.
Higher crossover frequencies can result in
instability.
Table 5: Compensation Values for Typical
Output Voltage/Capacitor Combinations
MP1583 Rev 2.0_06/30/04
Monolithic Power Systems
V
2.5V
3.3V
5V
12V
2.5V
3.3V
5V
12V
2.5V
3.3V
5V
12V
OUT
system
C5
22µF Ceramic
22µF Ceramic
22µF Ceramic
22µF Ceramic
47µF SP-Cap
47µF SP-Cap
47µF SP-Cap
47µF SP-Cap
560µF/6.3V
(30mΩ ESR)
560µF/6.3V
(30mΩ ESR)
470µF/10V
(30mΩ ESR)
220µF/25V
(30mΩ ESR)
frequencies
f
f
Z1
P3
= 1 / (2π*R3*C3)
= 1 / (2π*R3*C6)
crossover
C
10KΩ
R3
3.9KΩ
4.7KΩ
7.5KΩ
10KΩ
8.2KΩ
10KΩ
10KΩ
10KΩ
10KΩ
10KΩ
10KΩ
, of 40KHz.
result
frequency
C3
3.9nF
3.3nF
2.2nF
2.7nF
1.8nF
1.8nF
2.7nF
5.6nF
15nF
18nF
27nF
27nF
in
C4
None
None
None
None
None
None
None
None
1.5nF
1.5nF
None
None
slower
Lower
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Choosing the Compensation Components
The values of the compensation components
given in Table 5 yield a stable control loop for
the output voltage and capacitor given.
optimize the compensation components for
conditions not listed in Table 5, use the
following procedure:
Choose the compensation resistor to set the
desired crossover frequency (See Figure 3).
Determine the value by the following equation:
Putting in the know constants and setting the
crossover frequency to the desired 40kHz:
The value of R3 is limited to 10KΩ to prevent
output overshoot at startup, so if the value
calculated for R3 is greater than 10KΩ, use
10KΩ.
frequency is less than the desired 40kHz, and
is calculated by:
or
Choose the compensation capacitor to set the
zero to ¼ of the crossover frequency.
Determine the value by the following equation:
if R3 is less than 10KΩ, or if R3 = 10KΩ use
the following equation:
C3 ≈ 1.08x10
R3 = 2π*C
f
C3 = 4C
C
C3 = 2 / π*R3*f
= R3*G
In this case, the actual crossover
R3 ≈ 6.8x10
-5
OUT
f
C
EA
C
OUT
OUT
Step Down Converter
≈ 5.9 / C
*V
*G
*V
OUT
V
CS
OUT
OUT
*V
/ (R3
C
FB
*f
7
≈ 1.59x10
OUT
C
C
/ (2π*C
OUT
/ (G
2
*V
*G
*V
OUT
EA
EA
MP1583
OUT
*G
*G
OUT
-5
CS
CS
3A, 23V
/ R3
*V
*V
*V
OUT
FB
FB
)
)
)
To
7