MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
MP1591 Rev. 1.8
www.MonolithicPower.com
8
7/22/2005
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© 2005 MPS. All Rights Reserved.
TM
To optimize the compensation components that
are not listed in Table 4, use the following
procedure.
Choose the compensation resistor to set the
desired crossover frequency. Determine the
value by the following equation:
REF
CS
EA
C
OUT
V
G
G
f
V
5
C
2
3
R
×
×
×
×
×
π
=
Putting in the know constants and setting the
crossover frequency to the desired 33KHz:
OUT
7
V
5
C
10
88
.
6
3
R
×
×
×
≅
Choose the compensation capacitor to set the
zero below one fourth of the crossover
frequency. Determine the value by the following
equation:
3
R
10
93
.
1
f
3
R
2
4
C
5
C
−
×
≈
×
×
π
>
Determine
if
the
second
compensation
capacitor, C3, is required. It is required if the
ESR zero of the output capacitor occurs at less
than four times the crossover frequency, or
1
f
R
5
C
8
C
ESR
≥
×
×
×
π
If this is the case, then add the second
compensation resistor. Determine the value by
the equation:
3
R
R
5
C
3
C
)
MAX
(
ESR
×
=
Where RESR(MAX) is the maximum ESR of the
output capacitor.
Example:
VOUT = 5V, C5 = 22µF Ceramic (ESR = 10mΩ)
R3 ≈ 6.88x107 (22x10-6) (5) = 7568Ω
Use the nearest standard value of 7.5kΩ.
C4 > 1.93x10-5 / 7.5K = 2.57nF
Use standard value of 2.7nF.
8π x C5 x RESR x fC = 0.22, which is less than 1.
Therefore, no second compensation capacitor
(C3) is required.
External Bootstrap Diode
It is recommended that an external bootstrap
diode be added when the system has a 5V
fixed input or the power supply generates a 5V
output. This helps improve the efficiency of the
regulator. The bootstrap diode can be a low
cost one such as IN4148 or BAT54.
MP1591
SW
BS
10nF
5V
MP1591_F02
Figure 2—External Bootstrap Diode
This diode is also recommended for high duty
cycle operation (when
IN
OUT
V
V
>65%) and high
output voltage (VOUT>12V) applications.
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