TM
MP2303 – 3A, 28V, 340KHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
INITIAL RELEASE – SPECIFICATIONS SUBJECT TO CHANGE
To optimize the compensation components for
conditions not listed in Table 2, 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
Soft-Start Capacitor
To reduce input inrush current during startup, a
programmable soft-start is provided by
connecting a capacitor (C4) from pin SS to
GND. The soft-start time is given by:
t
SS
=
C4
×
0.8 V
6
µ
A
Where f
C
is the desired crossover frequency,
34KHz.
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. Determine the C3 value by the
following equation:
4
C3
>
2
π ×
R3
×
f
C
To reduce the susceptibility to noise, do not
leave SS pin open. Use a capacitor with small
value if you do not need soft-start function.
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
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 340KHz switching
frequency, or the following relationship is valid:
f
1
<
S
2
π ×
C2
×
R
ESR
2
BS
MP2303
SW
10nF
MP2303_F02
Figure 2—External Bootstrap Diode
This diode is also recommended for high duty
cycle operation (
V
OUT
>65%) and high output
V
IN
If this is the case, then add the optional
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
voltage (V
OUT
>12V) applications.
MP2303 Rev. 0.92
5/2/2006
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11
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