MP1593 – 3A, 28V, 385kHz STEP-DOWN CONVERTER
Table 3—Compensation Values for Typical
Output Voltage/Capacitor Combinations
Determine C3 by the following equation:
4
C3
VOUT
L
C2
R3
C3
C6
2 R3 fC
100μF
Ceramic
1.8V
4.7μH
5.6kΩ 3.3nF None
3.9kΩ 5.6nF None
5.6kΩ 8.2nF None
7.5kΩ 10nF None
10kΩ 3.3nF None
5.6kΩ 3.3nF 100pF
4.7kΩ 5.6nF None
6.8kΩ 10nF None
10kΩ 10nF None
10kΩ 5.6nF 1.5nF
10kΩ 8.2nF 1.5nF
Where R3 is the compensation resistor value.
4.7-
6.8μH
47μF
Ceramic
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:
2.5V
3.3V
5V
6.8-
10μH
22μFx2
Ceramic
10-
15μH
22μFx2
Ceramic
fS
2
1
15-
22μH
22μFx2
Ceramic
12V
1.8
2 C2 RESR
100μF
SP-CAP
4.7μH
Where C2 is the output capacitance value, RESR is
the ESR value of the output capacitor and fS is the
switching frequency. If this is the case, then add
the second compensation capacitor (C6) to set
the pole fP3 at the location of the ESR zero.
Determine C6 by the equation:
4.7-
6.8μH
47μF
SP-CAP
2.5V
3.3V
5V
6.8-
10μH
47μF
SP-CAP
10-
15μH
47μF
SP CAP
C2 RESR
4.7-
560μF Al.
6.8μH 30mΩ ESR
C6
2.5V
3.3V
5V
R3
6.8-
10μH
560μF Al
30mΩ ESR
Where C2 is the output capacitance value, RESR is
the ESR value of the output capacitor and R3 is
the compensation resistor.
10-
15μH
470μF Al.
30mΩ ESR
15kΩ 5.6nF
1nF
15-
22μH
220μF Al.
30mΩ ESR
PCB Layout Guide
12V
15kΩ 4.7nF 390pF
PCB layout is very important to achieve stable
operation. It is highly recommended to duplicate
EVB layout for optimum performance.
To optimize the compensation components for
conditions not listed in Table 3, the following
procedure can be used.
If change is necessary, please follow these
guidelines and take Figure2 and 3 for references.
1. Choose the compensation resistor (R3) to set
the desired crossover frequency. Determine R3
by the following equation:
1) Keep the path of switching current short
and minimize the loop area formed by Input
cap, high-side MOSFET and low-side
MOSFET/schottky diode.
2 C2 fC VOUT
R3
GEA GCS
VFB
2) Keep
the
connection
of
low-side
Where fC is the desired crossover frequency
(which typically has a value no higher than
38KHz).
MOSFET/schottky diode between SW pin
and input power ground as short and wide
as possible.
2. Choose the compensation capacitor (C3) to
achieve the desired phase margin. For
applications with typical inductor values, setting
the compensation zero, fZ1, below one forth of
the crossover frequency provides sufficient
phase margin.
3) Bypass ceramic capacitors are suggested
to be put close to the VIN and VCC Pin.
4) Ensure all feedback connections are short
and direct. Place the feedback resistors
and compensation components as close to
the chip as possible.
MP1593 Rev. 2.11
1/10/2013
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