MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
To optimize the compensation components that
are not listed in Table 4, use the following
procedure.
Example:
VOUT = 5V, C5 = 22µF Ceramic (ESR = 10mꢀ)
R3 ≈ 6.88x107 (22x10-6) (5) = 7568ꢀ
Use the nearest standard value of 7.5kꢀ.
C4 > 1.93x10-5 / 7.5K = 2.57nF
Use standard value of 2.7nF.
Choose the compensation resistor to set the
desired crossover frequency. Determine the
value by the following equation:
2π × C5 × VOUT × fC
R3 =
GEA × GCS × VREF
8π x C5 x RESR x fC = 0.22, which is less than 1.
Therefore, no second compensation capacitor
(C3) is required.
Putting in the know constants and setting the
crossover frequency to the desired 33KHz:
R3 ≅ 6.88 ×107 × C5 × VOUT
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.
Choose the compensation capacitor to set the
zero below one fourth of the crossover
frequency. Determine the value by the following
equation:
2
1.93 ×10−5
C4 >
≈
5V
π × R3 × fC
R3
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
BS
10nF
MP1591
3
SW
8π × C5 × RESR × fC ≥ 1
Figure 2—External Bootstrap Diode
If this is the case, then add the second
compensation resistor. Determine the value by
the equation:
This diode is also recommended for high duty
VOUT
cycle operation (when
>65%) and high
VIN
C5 × RESR(MAX)
C3 =
output voltage (VOUT>12V) applications.
R3
Where RESR(MAX) is the maximum ESR of the
output capacitor.
MP1591 Rev. 2.3
9/27/2006
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