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| 型号： | MP28275EL |
| 厂商： | MONOLITHIC POWER SYSTEMS INC |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, 1300 kHz SWITCHING FREQ-MAX, PDSO14 |
| 封装： | 3 X 4 MM, MO-229VGED-3, QFN-14 |
| 文件页数： | 3/14页 |
| 文件大小： | 372K |
| 代理商： | MP28275EL |
MP28275 – 5A, 7V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
MP28275 Rev. 0.91
www.MonolithicPower.com
11
12/2/2009
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
2009 MPS. All Rights Reserved.
APPLICATION INFORMATION
Setting the Output Voltage
The external resistor divider is used to set the
output voltage (see Typical Application on page
1). The feedback resistor R1 also sets the
feedback loop bandwidth with the internal
compensation capacitor (see Typical Application
on page 1). Choose R1 to be around 40.2k for
optimal transient response. R2 is then given by:
OUT
FB
R1
R2
V
1
V
=
The T-type network is highly recommended when
Vo is low, as Figure 6 shows.
FB 1
R2
R1
Rt
VOUT
Figure 6— T-type Network
Table 1 lists the recommended T-type resistors
value for common output voltages.
Table 1—Resistor Selection for Common
Output Voltages
VOUT (V)
R1 (k)
R2 (k)
Rt (k)
1.05
4.99(1%)
16.5(1%)
24.9(1%)
1.2
4.99(1%)
10.2(1%)
24.9(1%)
1.5
4.99(1%)
5.76(1%)
24.9(1%)
1.8
4.99(1%)
4.02(1%)
24.9(1%)
2.5
40.2 (1%)
19.1(1%)
0
3.3
40.2(1%)
13(1%)
0
Selecting the Inductor
A 1H to 10H inductor with a DC current rating
of at least 25% percent higher than the maximum
load
current
is
recommended
for
most
applications. For highest efficiency, the inductor
DC resistance should be less than 15m. For
most designs, the inductance value can be
derived from the following equation.
OSC
L
IN
OUT
IN
OUT
f
I
V
)
V
(
V
L
×
Δ
×
×
=
Where ΔIL is the inductor ripple current.
Choose
inductor
ripple
current
to
be
approximately 30% if the maximum load current,
5A. The maximum inductor peak current is:
2
I
L
LOAD
)
MAX
(
L
Δ
+
=
Under light load conditions below 100mA, larger
inductance
is
recommended
for
improved
efficiency.
Selecting the Input Capacitor
The input current to the step-down converter is
discontinuous, therefore a capacitor is required to
supply the AC current to the step-down converter
while maintaining the DC input voltage. Use low ESR
capacitors for the best performance. Ceramic
capacitors with X5R or X7R dielectrics are highly
recommended because of their low ESR and
small
temperature
coefficients.
For
most
applications, a 22F capacitor is sufficient.
Since the input capacitor (C1) absorbs the input
switching current it requires an adequate ripple
current rating. The RMS current in the input capacitor
can be estimated by:
×
×
=
IN
OUT
IN
OUT
LOAD
1
C
V
1
V
I
The worse case condition occurs at VIN = 2VOUT,
where:
2
I
LOAD
1
C =
For simplification, choose the input capacitor
whose RMS current rating greater than half of the
maximum load current.
The input capacitor can be electrolytic, tantalum
or ceramic. When using electrolytic or tantalum
capacitors,
a
small,
high
quality
ceramic
capacitor, i.e. 0.1μF, should be placed as close
to the IC as possible. When using ceramic
capacitors, make sure that they have enough
capacitance to provide sufficient charge to
prevent excessive voltage ripple at input. The
input voltage ripple caused by capacitance can
be estimated by:
×
=
Δ
IN
OUT
IN
OUT
S
LOAD
IN
V
1
V
1
C
f
I
V
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