MP1593 – 3A, 28V, 385KHz STEP-DOWN CONVERTER
Table 3—Compensation Values for Typical
Output Voltage/Capacitor Combinations
V
OUT
1.8V
2.5V
3.3V
5V
12V
1.8
2.5V
3.3V
5V
2.5V
3.3V
5V
12V
Determine the C3 value by the following equation:
C3
>
4
2
π ×
R3
×
f
C
L
4.7µH
4.7-
6.8µH
6.8-
10µH
10-
15µH
15-
22µH
4.7µH
4.7-
6.8µH
6.8-
10µH
10-
15µH
4.7-
6.8µH
6.8-
10µH
10-
15µH
15-
22µH
C2
100µF
Ceramic
47µF
Ceramic
22µFx2
Ceramic
22µFx2
Ceramic
22µFx2
Ceramic
100µF
SP-CAP
47µF
SP-CAP
47µF
SP-CAP
47µF
SP CAP
560µF Al.
30mΩ ESR
560µF Al
30mΩ ESR
470µF Al.
30mΩ ESR
220µF Al.
30mΩ ESR
R3
5.6kΩ
3.9kΩ
5.6kΩ
7.5kΩ
10kΩ
5.6kΩ
4.7kΩ
6.8kΩ
10kΩ
10kΩ
10kΩ
15kΩ
15kΩ
C3
3.3nF
5.6nF
8.2nF
10nF
3.3nF
C6
None
None
None
None
None
Where, R3 is the compensation resistor value.
3. Determine if the second compensation
capacitor (C6) is required. It is required if the ESR
zero of the output capacitor is located at less than
half of the 385KHz switching frequency, or the
following relationship is valid:
f
1
<
S
2
π ×
C2
×
R
ESR
2
3.3nF 100pF
5.6nF
10nF
10nF
5.6nF
8.2nF
5.6nF
None
None
None
1.5nF
1.5nF
1nF
Where, C
O
is the output capacitance value, R
ESR
is the ESR value of the output capacitor, and f
S
is
the 385KHz switching frequency. If this is the
case, then add the second compensation
capacitor (C6) to set the pole f
P3
at the location of
the ESR zero. Determine the C6 value by the
equation:
C6
=
C2
×
R
ESR
R3
4.7nF 390pF
Where, C
O
is the output capacitance value, R
ESR
is the ESR value of the output capacitor, and R3
is the compensation resistor.
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.
5V
To optimize the compensation components for
conditions not listed in Table 3, the following
procedure can be used.
1. Choose the compensation resistor (R3) to set
the desired crossover frequency. Determine the
R3 value by the following equation:
R3
=
2
π ×
C2
×
f
C
V
OUT
×
G
EA
×
G
CS
V
FB
BS
Where f
C
is the desired crossover frequency
(which typically has a value no higher than
38KHz).
2. Choose the compensation capacitor (C3) to
achieve the desired phase margin. For
applications with typical inductor values, setting
the compensation zero, f
Z1
, below one forth of
the crossover frequency provides sufficient
phase margin.
MP1593
SW
10nF
MP1593_F02
Figure 2—External Bootstrap Diode
This diode is also recommended for high duty
cycle operation (when
V
OUT
>65%) and high
V
IN
output voltage (V
OUT
>12V) applications.
MP1593 Rev. 1.8
1/6/2006
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