TM
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
Choose the compensation resistor to set the
desired crossover frequency. Determine the
value by the following equation:
Use the nearest standard value of 7.5kꢀ.
C4 > 1.93x10-5 / 7.5K = 2.57nF
Use standard value of 2.7nF.
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
BS
10nF
MP1591
SW
8π × C5 × RESR × fC ≥ 1
MP1591_F02
If this is the case, then add the second
compensation resistor. Determine the value by
the equation:
Figure 2—External Bootstrap Diode
This diode is also recommended for high duty
VOUT
cycle operation (when
>65%) and high
C5 × RESR(MAX)
VIN
C3 =
R3
output voltage (VOUT>12V) applications.
Where RESR(MAX) is the maximum ESR of the
output capacitor.
MP1591 Rev. 2.1
12/5/2005
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