参数资料
型号: MAX1962
厂商: Maxim Integrated Products, Inc.
英文描述: 2.35V to 5.5V, 0.5% Accurate, 1MHz PWM Step-Down Controllers with Voltage Margining
中文描述: 2.35V至5.5V、0.5%精度、1MHz PWM降压型控制器,带有电压裕量控制
文件页数: 15/29页
文件大小: 455K
代理商: MAX1962
Output Capacitor Selection
The output filter capacitor must have low enough effective
series resistance (ESR) to meet output ripple and load
transient requirements. In addition, the capacitance value
must be high enough to absorb the inductor energy
during load steps.
In applications where the output is subject to large load
transients, low ESR is needed to prevent the output
from dipping too low (V
DIP
) during a load step:
In applications with less severe load steps, maximum
ESR may be governed by what is needed to maintain
acceptable output voltage ripple:
To satisfy both load step and ripple requirements,
select the lowest value from the above two equations.
The capacitor is usually selected by physical size, ESR,
and voltage rating, rather than by capacitance value.
With current tantalum, electrolytic, and polymer capaci-
tor technology, the bulk capacitance will also be suffi-
cient once the ESR requirement is satisfied.
When using low-capacity filter capacitors such as
ceramic, capacitor size is usually determined by the
capacitance needed to prevent voltage undershoot
and overshoot during load transients. The overshoot
voltage (V
SOAR
) is given by:
Generally, once enough capacitance is in place to meet
the overshoot requirement, undershoot at the rising load
edge is no longer a problem.
Input Capacitor Selection
The input capacitor (C
IN
) reduces the current peaks
drawn from the input supply and reduces noise injec-
tion. The source impedance to the input supply largely
determines the value of C
IN
. High source impedance
requires high input capacitance. The input capacitor
must meet the ripple current requirement (I
RMS
)
imposed by the switching currents.
The RMS input ripple current is given by:
For optimal circuit reliability, choose a capacitor that
has less than 10
°
C temperature rise at the peak ripple
current.
Compensation and Stability
Compensation with Ceramic Output Capacitors
The high switching frequency range of the
MAX1960/MAX1961/MAX1962 allows the use of ceramic
output capacitors. Since the ESR of ceramic capacitors
is very low typically, the frequency of the associated
transfer function zero is higher than the unity-gain
crossover frequency and the zero cannot be used to
compensate for the double pole created by the output
inductor and capacitor. The solution is Type 3 compen-
sation (Figure 5), which takes advantage of local feed-
back to create two zeros and three poles (Figure 6). The
frequency of the poles and zeros are described below:
Unity-gain crossover frequency:
f
R
C
V
V
L
C
IN MAX
(
RAMP
0
0
0
1
3
1
2
=
×
×
×
×
×
)
π
f
R
C
ZESR
ESR
=
×
×
1
2
0
π
f
R
R
C
Z2
1
+
2
2
3
3
=
π
(
)
f
R
C
Z1
1
2
1
1
=
×
×
π
f
L
C
LC
=
×
1
2
0
0
π
f
R
C
C
C
C
P3
1
2
1
1
1
2
2
=
×
×
×
+
π
f
R
C
P2
1
2
2
3
=
×
×
π
f
P1
0
=
I
I
V
V
V
V
RMS
LOAD
OUT
IN
OUT
IN
=
×
×
(
)
-
V
L
I
V
C
SOAR
PEAK
×
OUT
OUT
=
×
(
)
×
2
2
R
V
LIR
I
ESR
RIPPLE P P
×
LOAD MAX
)
(
)
R
V
I
ESR
DIP
LOADSTEP MAX
(
)
M
2.35V to 5.5V, 0.5% Accurate, 1MHz PWM
Step-Down Controllers with Voltage Margining
______________________________________________________________________________________
15
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