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參數(shù)資料
| 型號(hào)： | ADP1147AR-3.3 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Current-Mode SMPS Controller |
| 中文描述： | SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | SOIC-8 |
| 文件頁(yè)數(shù)： | 9/12頁(yè) |
| 文件大?。?/td> | 195K |
| 代理商： | ADP1147AR-3.3 |
ADP1147-3.3/ADP1147-5
–9–
REV. 0
T he Schottky diode is in conduction during the MOSFET off-
time. A short circuit of V
OUT
= 0 is the most demanding situa-
tion on for the diode. During this time it must be capable of
delivering I
SC(PK )
for duty cycles approaching 100%. T he equa-
tion below is used to calculate the average current conducted by
the diode under normal load conditions.
I
D
1
=
V
IN
±
V
OUT
V
IN
+
V
D
×
I
LOAD
T o guard against increased power dissipation due to undesired
ringing, it is extremely important to adhere to the following:
1. Use proper grounding techniques.
2. K eep all track lengths as short as possible, especially connec-
tions made to the diode (refer to PCB Layout Considerations
section).
T he allowable forward voltage drop of the diode is determined
by the maximum short circuit current and power dissipation.
T he equation below is used to calculate V
F
:
V
F
=
P
D
/I
SC(PK)
where
P
D
is the maximum allowable power dissipation and is
determined by the system efficiency and thermal requirements
(refer to Efficiency Section).
C
IN
Considerations
During the continuous mode of operation the current drawn
from the source is a square wave with a duty cycle equal to
V
OUT
/V
IN
. T o reduce or prevent large voltage transients an input
capacitor with a low ESR value and capable of handling the
maximum rms current should be selected. T he formula below
is used to determine the required maximum rms capacitor
current:
C
IN
I
RMS
=
[
V
OUT
(
V
IN
–V
OUT
)]
0.5
×
I
MAX
/
V
IN
T he maximum for this formula is reached when V
IN
= 2 V
OUT
,
where I
RMS
= I
OUT
/2. It is best to use this worst case scenario for
design margin. Manufacturers of capacitors typically base the
current ratings of their caps on a 2000-hour life. T his requires a
prudent designer to use capacitors that are derated or rated at a
higher temperature. T he use of multiple capacitors in parallel
may also be used to meet design requirements. T he capacitor
manufacturer should be consulted for questions regarding spe-
cific capacitor selection.
In addition, for high frequency decoupling a 0.1
μ
F to 1.0
μ
F
ceramic capacitor should be placed and connected as close to
the V
IN
pin as possible.
C
OUT
Considerations
T he minimum required ESR value is the primary consideration
when selecting C
OUT
. For proper circuit operation the ESR
value of C
OUT
must be less than two times the value selected for
R
SENSE
(see equation below):
C
OUT
Minimum Required ESR
< 2
R
SENSE
When selecting a capacitor for C
OUT
, the minimum required
ESR is the primary concern. Proper circuit operation mandates
that the ESR value of C
OUT
must be less than two times the
value of R
SENSE
.
A capacitor with an ESR value equal to R
SENSE
will provide the
best overall efficiency. If the ESR value of C
OUT
increases to
two times R
SENSE
a 1% decrease in efficiency results. United
Chemicon, Nichicon and Sprague are three manufacturers of
high grade capacitors. Sprague offers a capacitor that uses an
OS-CON semiconductor dielectric. T his style capacitor pro-
vides the lowest amount of ESR for its size, but at a higher cost.
Most capacitors that meet the ESR requirements for I
P-P
ripple
will usually meet or exceed the rms current requirements. The
specifications for the selected capacitor should be consulted.
Surface mount applications may require the use of multiple
capacitors in parallel to meet the ESR or rms current require-
ments. If dry tantalum capacitors are used it is critical that they
be surge tested and recommended by the manufacturer for use
in switching power supplies such as T ype 593D from Sprague.
AVX offers the T PS series of capacitors with various heights
from 2 mm to 4 mm. T he manufacturer should be consulted
for the latest information, specifications and recommendations
concerning specific capacitors. When operating with low supply
voltages, a minimum output capacitance will be required to
prevent the device from operating in a low frequency mode (see
Figure 5). T he output ripple also increases at low frequencies if
C
OUT
is too small.
T ransient Response
T he response of the regulator loop can be verified by monitoring
the transient load response. Several cycles may be required for a
switching regulator circuit to respond to a step change in the dc
load current (resistive load). When a step in the load current
takes place a change in V
OUT
occurs. T he amount of the change
in V
OUT
is equal to the delta of I
LOAD
×
ESR of C
OUT
. T he delta
of I
LOAD
charges or discharges the output voltage on capacitor
C
OUT
. T his continues until the regulator loop responds to the
change in load and is able to restore V
OUT
to its original value.
V
OUT
should be monitored during the step change in load for
overshoot, undershoot or ringing, which may indicate a stability
problem. T he circuit shown in Figure 1 contains external com-
ponents that should provide sufficient compensation for most
applications. T he most demanding form of a transient that can
be placed on a switching regulator is the hot switching in of
loads that contain bypass or other sources of capacitance greater
than 1
μ
F. When a discharged capacitor is placed on the load it
is effectively placed in parallel with the output cap C
OUT
, and
results in a rapid drop in the output voltage V
OUT
. Switching
regulators are not capable of supplying enough instantaneous
current to prevent this from occurring. T herefore, the inrush
current to the load capacitors should be held below the current
limit of the design.
E fficiency
Efficiency is one of the most important reasons for choosing a
switching regulator. T he percentile efficiency of a regulator can
be determined by dividing the output power of the device by the
input power and then multiplying the results by 100. Efficiency
losses can occur at any point in a circuit and it is important to
analyze the individual losses to determine changes that would
yield the most improvement. T he efficiency of a circuit can be
expressed as:
% efficiency =
100
% –
(
% L
1
+ % L
2
+ % L
3
. . . etc.
)
L1, L2, L3, etc., are the individual losses as a percentage of the
input power. In high efficiency circuits small errors result when
expressing losses as a percentage of the output power.
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| ADP1147AR-5 | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
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| ADP1148-3.3 | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:High Efficiency Synchronous Step-Down Switching Regulators(273.94 k) |
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