MP2351DQ MPS [Monolithic Power Systems], MP2351DQ Datasheet - Page 7

• Manufacturer Part Number: MP2351DQ
• Description: 1.5A, 23V, 1.4MHz Step-Down Converter
• Manufacturer: MPS [Monolithic Power Systems]

In this case, a third pole set by the
compensation
compensation
compensate the effect of the ESR zero on the
loop gain. This pole is located at:
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.
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
switching frequency.
To optimize the compensation components, 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:
Where f
which is typically less than one tenth of the
switching frequency.
2. Choose the compensation capacitor (C3) to
achieve
applications with typical inductor values, setting
the compensation zero, f
of the crossover frequency provides sufficient
phase margin. Determine the C3 value by the
following equation:
Where R3 is the compensation resistor.
MP2351 Rev. 1.5
1/6/2006
C
the
is the desired crossover frequency,
R
TM
3
f
=
desired
C
P
resistor
3
3
capacitor
2
G
π
=
>
EA
×
2
2
C
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
π
×
π
2
×
G
×
×
C
R
CS
Z1
phase
4
1
f
6
3
C
(R3)
, to below one forth
×
×
×
R
f
(C6)
C
V
3
V
OUT
FB
is
margin.
and
used
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www.MonolithicPower.com
MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER
For
the
to
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:
If this is the case, then add the second
compensation capacitor (C6) to set the pole f
at the location of the ESR zero. Determine the
C6 value by the equation:
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.
This diode is also recommended for high duty
cycle operation (when
output voltage (V
Figure 2—External Bootstrap Diode
MP2351
2
π
C
OUT
×
6
C
=
>12V) applications.
2
1
SW
BS
C
×
2
R
×
ESR
R
V
R
V
3
OUT
IN
ESR
MP2351_F02
<
5V
10nF
>65%) and high
f
2
S
P3
7