TM
MP28372 — DUAL 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER
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 switching frequency, or the
following relationship is valid:
1
fESR
=
2π × C2× RESR
In this case (as shown in Figure 2), a third pole
set by the compensation capacitor (C6) and the
compensation resistor (R3) is used to
compensate the effect of the ESR zero on the
loop gain. This pole is located at:
fS
2
1
<
2π × C2× RESR
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 the
C6 value by the equation:
1
fP3
=
2π × C6 × R3
The goal of compensation design is to shape
the converter transfer function to get a desired
loop gain. The system crossover frequency
where the feedback loop has the unity gain is
important.
C2 × RESR
C6 =
R3
Soft-Start
Lower crossover frequencies result in slower
line and load transient responses, while higher
crossover frequencies could cause system
unstable. A good rule of thumb is to set the
crossover frequency to below one-tenth of the
Each channel is soft-start controlled with the
SSA and SSB pins. Use capacitors to control
the ramp time using the equation:
RampTime = 2.2× 9kΩ × C4
switching
frequency.
To
optimize
the
External Bootstrap Diode
compensation components for conditions not
listed in Table 2, the following procedure can be
used:
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.
1. Choose the compensation resistor (R3) to set
the desired crossover frequency. Determine the
R3 value by the following equation:
5V
2π × C2× fC VOUT
R3 =
×
GEA × GCS
VFB
BSA/B
Where fC is the desired crossover frequency,
which is typically less than one tenth of the
switching frequency.
10nF
MP28372
SWA/B
2. Choose the compensation capacitor (C3) to
achieve the desired phase margin. For
applications with typical inductor values, setting
the compensation zero, fZ1, to below one forth
of the crossover frequency provides sufficient
phase margin. Determine the C3 value by the
following equation:
Figure 2—External Bootstrap Diode
This diode is also recommended for high duty
VOUT
cycle operation (when
>65%) and high
VIN
output voltage (VOUT>12V) applications.
4
C3 >
2π × R3 × fC
Where R3 is the compensation resistor value.
MP28372 Rev. 1.4
12/10/2007
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