TM
MP1411 – 2A, 18V, 380KHz STEP-DOWN CONVERTER
In this case, 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:
f
P3
=
1
2
π ×
C6
×
R3
If this is the case, then add the second
compensation capacitor (C6) to set the pole f
P3
at
the location of the ESR zero. Determine the C6
value by the equation:
C6
=
C
O
×
R
ESR
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.
Lower crossover frequencies result in slower line
and load transient responses, while higher
crossover frequencies could cause the system to
become 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:
2
π ×
C
O
×
f
C
V
OUT
R3
=
×
G
EA
×
G
CS
V
FB
Power Dissipation and Temperature Rise
The power dissipation of the MP1411 is mostly
from the conduction loss of the internal main
switch. This power loss is estimated to be:
P
LOSS
≅
V
OUT
2
×
I
OUT
×
0.18
Ω ×
1.3
V
IN
Where 1.3 is a temperature coefficient factor that
reflects the increase in the R
DS(ON)
resistance at
elevated temperatures.
For example: for V
IN
= 12V, V
OUT
= 3.3V and I
OUT
= 2A:
P
LOSS
≅
3 .3 V
×
(2A )
2
×
0.18
Ω ×
1.3
=
0.26 W
12V
Where f
C
is the desired crossover frequency,
which is typically less than one tenth of the
switching frequency.
2. Choose the compensation capacitor (C3) to
achieve the desired phase margin. For
applications with typical inductor values, setting
the compensation zero, f
Z1
, to below one forth of
the crossover frequency provides sufficient
phase margin. Determine the C3 value by the
following equation:
C3
>
2
π ×
R3
×
f
C
Because the thermal resistance
θ
JA
is 105°C/W,
the resulting rise in temperature between junction
and ambient is approximately 27°C. Therefore,
caution must be exercised when using the
MP1411 in applications with high duty cycles.
PCB Layout Guide
PCB layout is very important to achieve stable
operation. Please follow these guidelines and
take Figure2 and 3 for references.
1)
Keep the path of switching current short and
minimize the loop area formed by Input cap,
high-side and low-side MOSFETs.
Keep the connection of low-side MOSFET
between SW pin and input power ground as
short and wide as possible.
Ensure all feedback connections are short
and direct. Place the feedback resistors and
compensation components as close to the
chip as possible.
Route SW away from sensitive analog areas
such as FB.
2)
Where R3 is the compensation resistor value.
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:
f
1
<
S
2
π ×
C
O
×
R
ESR
2
3)
4)
MP1411 Rev. 1.3
1/22/2010
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