B340L MPS [Monolithic Power Systems], B340L Datasheet - Page 7

• Manufacturer Part Number: B340L
• Description: 2A, 32V, 330KHz Step-Down Converter
• Manufacturer: MPS [Monolithic Power Systems]

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:
Where V
1.230V, A
amplifier voltage gain, 400 V/V, and G
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:
Where f
amplifier transconductance (770µS) and
The system has one zero of importance due to
the compensation capacitor (C4) and the
compensation resistor (R3) which 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 C3
MP1591 Rev. 2.3
9/27/2006
A
P1
VDC
REF
is the first pole, and G
VEA
is the feedback threshold voltage,
=
f
f
P
P
2
f
f
V
V
1
is the transconductance error
P
Z
OUT
1
3
REF
=
=
=
=
2 (
2 (
2 (
2 (
×
π
π
π
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
×
A
×
π
R
×
×
G
A
VEA
R
LOAD
R
MEA
VEA
1
1
1
3
3
×
×
×
G
×
C
×
C
C
CS
) 4
) 3
C
) 4
) 5
MEA
×
R
LOAD
is the error
© 2006 MPS. All Rights Reserved.
CS
www.MonolithicPower.com
is the
MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
The system crossover frequency f
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
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.
Note: “AL” = Electrolytic
Table 4—Compensation Values for Typical
V
2.5V
3.3V
2.5V
3.3V
2.5V
3.3V
12V
12V
12V
Output Voltage/Capacitor Combinations
5V
5V
5V
OUT
560µF/6.3V, AL
560µF/6.3V, AL
220µF/25V, AL
470µF/10V, AL
22µF Ceramic
22µF Ceramic
22µF Ceramic
22µF Ceramic
47µF SP-Cap
47µF SP-Cap
47µF SP-Cap
47µF SP-Cap
30mΩ ESR
30mΩ ESR
30mΩ ESR
30mΩ ESR
frequency
C5
100kΩ
120kΩ
150kΩ
180kΩ
3.9kΩ
5.1kΩ
7.5kΩ
8.2kΩ
10kΩ
16kΩ
36kΩ
18kΩ
of
R3
33KHz.
150pF
120pF
None
None
None
None
None
None
None
None
82pF
33pF
C3
C,
4.7nF
3.9nF
2.7nF
1.2nF
2.2nF
2.2nF
1.5nF
Lower
1nF
1nF
1nF
1nF
1nF
C4
(the
7