MP9141 – 1.8A STEP-DOWN DC TO DC CONVERTER
MPS CONFIDENTIAL AND PROPRIETARY INFORMATION– INTERNAL USE ONLY
The system has one zero of importance, due to
the compensation capacitor (C5) and the
compensation resistor (R1). This is:
Choose the compensation resistor to set the
desired crossover frequency. Determine the
value by the following equation:
f Z1
=
1
2π × R1× C5
If a large value capacitor (C7) with relatively
high equivalent-series-resistance (ESR) is
used, the zero due to the capacitance and ESR
of the output capacitor can be compensated by
a third pole set by R1 and C4:
fP3
=
1
2π × R1× C4
The system crossover frequency (the frequency
where the loop gain drops to 1 or 0dB) is
important. A good standard is to set the
crossover frequency to approximately one tenth
of the switching frequency. In this case, the
switching frequency is 380KHz. Therefore, use
a crossover frequency (fC) of 40KHz. 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 capacitor given.
Table 4—Compensation Values for Typical
Output Voltage/Capacitor Combinations
VOUT
C7
R1
C5 C4
2.5V 22µF Ceramic 7.5kΩ 2.2nF None
3.3V 22µF Ceramic 10kΩ 1.5nF None
5V 22µF Ceramic 10kΩ 2.2nF None
12V 22µF Ceramic 10kΩ 5.6nF None
2.5V
560µF/6.3V
(30mΩ ESR)
10kΩ 30nF None
3.3V
560µF/6.3V
(30mΩ ESR)
10kΩ 39nF None
5V
470µF/10V
(30mΩ ESR)
10kΩ 47nF None
12V
220µF/25V
(30mΩ ESR)
10kΩ 56nF None
To optimize the compensation components for
conditions not listed in Table 4, use the
following procedure:
R1
=
2π
×
C7
×
VOUT
×
(GEA
×
fC
GCS
×
VFB
)
Putting in the known constants and setting the
crossover frequency to the desired 40KHz:
R1 ≈ 1.37 × 108 × C7 × VOUT
The value of R1 is limited to 10kΩ to prevent
output overshoot at startup. Therefore, if the value
calculated for R1 is greater than 10kΩ, use 10kΩ.
In this case, the actual crossover frequency is
less than the desired 40KHz, and is calculated by:
fC
=
R1× GEA × GCS × VFB
2π × C7 × VOUT
or:
fC
≈
2.92 × 10 −4 × R1
C7 × VOUT
Choose the compensation capacitor to set the
zero to one fourth of the crossover frequency.
Determine the value by the following equation:
C5 = 0.22 × C7 × VOUT
R1
Determine if the second compensation
capacitor, C4 is required. It is required if the
ESR zero of the output capacitor occurs at less
than four times the crossover frequency.
8π × C7 × RESR × fC ≥ 1
or:
7.34 × 10 −5 × R1× RESR ≥ 1
VOUT
Where RESR is the equivalent series resistance
of the output capacitor. If this is the case, add
the second compensation capacitor. Determine
the value by the equation:
C4 = C7 × RESR(MAX)
R1
Where RESR(MAX) is the maximum ESR of the
output capacitor.
MP9141 Rev. 1.11
www.MonolithicPower.com
8
4/18/2011
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2011 MPS. All Rights Reserved.