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參數(shù)資料
| 型號： | ADP1147 |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 抗輻射高效，5安培開關穩(wěn)壓器 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大?。?/td> | 195K |
| 代理商： | ADP1147 |
ADP1147-3.3/ADP1147-5
–8–
REV. 0
T he formula used to calculate the continuous operating fre-
quency is:
f
=
1
V
OUT
+
V
D
V
IN
+
V
D
t
OFF
t
OFF
=
1.3
×
10
4
×
C
T
×
V
REG
V
OUT
V
REG
is the value of the desired output voltage.
V
OUT
is the ac-
tual measured value of the output voltage. When in regulation
V
REG
/V
OUT
is equal to 1. T he switching frequency of the ADP1147
decreases as the input voltage decreases. T he ADP1147 will
reduce the t
OFF
time by increasing the discharge current in ca-
pacitor C
T
if the input to output voltage differential falls below
1.5 volts. T his is to eliminate the possible occurrence of audible
switching prior to dropout.
Now that the operating frequency has been determined and the
value selected for C
T
, the required inductance for inductor L
can be computed. T he inductor L should be chosen so it will
generate no more than 25 mV/R
SENSE
of peak-to-peak inductor
ripple current.
T he following equation is used to determine the required value
for inductor L:
25
mV
R
SENSE
=
(
V
OUT
+
V
D
)
×
t
OFF
L
MIN
or
L
MIN
=
(
V
OUT
+
V
D
)
×
t
OFF
×
R
SENSE
25
mV
Substituting for t
OFF
above gives the minimum required induc-
tor value of:
L
MIN
= 5.1
×
10
5
×
R
SENSE
×
C
T
×
V
REG
T he ESR requirements for the output storage capacitor can be
relaxed by increasing the inductor value, but efficiency due to
copper losses will be reduced. Conversely, the use of too low an
inductance may allow the inductor current to become discon-
tinuous, causing the device to enter the power savings mode
prematurely. As a result of this the power savings threshold is
lowered and the efficiency at lower current levels is severely
reduced.
Inductor Core Considerations
Now that the minimum inductance value for L has been deter-
mined, the inductor core selection can be made. High efficiency
converters generally cannot afford the core losses found in low
cost powdered iron cores. T his forces the use of a more expen-
sive ferrite, molypermalloy, or K ool Mu
cores. T he typical
efficiency in Figure 1 reflects the use of a molypermalloy core.
T he cost of the inductor can be cut in half by Using a K ool Mu
core type CT X 50-4 by Coiltronics, but the efficiency will be
approximately 1%–2% less. T he actual core losses are not de-
pendent on the size of the core, but on the amount of induc-
tance. An increase in inductance will yield a decrease in the
amount of core loss. Although this appears to be desirable, more
inductance requires more turns of wire with added resistance
and greater copper losses.
K ool Mu is a registered trademark of Magnetics, Inc.
Using a ferrite cores in a design can produce very low core
losses, allowing the designer to focus on minimizing copper loss
and core saturation problems. Ferrite cores exhibit a condition
known as “Hard Saturation,” which results in an abrupt collapse
of the inductance when the peak design current is exceeded.
T his causes the inductor ripple current to rise sharply, the out-
put ripple voltage to increase and the power savings mode of
operation to be erroneously activated. T o prevent this from
occurring the core should never be allowed to saturate.
Molypermalloy (from Magnetics, Inc.) is a very good, low loss
core material for a toroids, but is more expensive than a ferrite
core. A reasonable compromise between price and performance,
from the same manufacturer is K ool Mu. T oroidal cores are
extremely desirable where efficient use of available space and
several layers of wire are required. T hey are available in various
surface mount configurations from Coiltronics Inc. and other
companies.
Power MOSFE T Selection and Considerations
T he ADP1147 requires the use of an external P-channel
MOSFET . T he major parameters to be considered when select-
ing the power MOSFET are the threshold voltage V
GS(T H)
and
the on resistance of the device R
DS(ON)
.
T he minimum input voltage determines if the design requires a
logic level or a standard threshold MOSFET . In applications
where the input voltage is > 8 volts, a standard threshold
MOSFET with a V
GS(T H)
of < 4 volts can be used. In designs
where V
IN
is < 8 volts, a logic level MOSFET with a V
GS(T H)
of
< 2.5 volts is recommended. Note: If a logic level MOSFET
is selected, the supply voltage to the ADP1147 must not
exceed the absolute maximum for the V
GS
of the MOSFET
(e.g., <
±
8 volts for IRF7304).
T he R
DS(ON)
requirement for the selected power MOSFET is
determined by the maximum output current (I
MAX
). An as-
sumption is made that when the ADP1147 is operating in the
continuous mode, either the Schottky Diode or the MOSFET
are always conducting the average load current. T he following
formulas are used to determine the duty cycle of each of the
components.
P
Channel MOSFET DutyCycle
=
V
OUT
+
V
D
V
IN
+
V
D
Schottky DiodeDutyCycle
=
V
IN
±
V
D
V
IN
+
V
D
Once the Duty Cycle is known, the R
DS(ON)
requirement for the
Power MOSFET can be determined by:
R
DS
(
ON
)
=
(
V
IN
+
V
D
)
×
P
P
(
V
OUT
+
V
D
)
×
I
MAX
2
×
(1
+δ
P
)
where P
P
is the max allowable power dissipation and where
δ
P
is
the temperature dependency of R
DS(ON)
for the MOSFET . Effi-
ciency and thermal requirements will determine the value of P
P
,
(refer to Efficiency section). MOSFET S usually specify the 1+
δ
as a normalized R
DS(ON )
vs. temperature trace, and
δ
can be
approximated to 0.007/
°
C for most low voltage MOSFET s.
Output Diode Considerations
When selecting the output diode careful consideration should be
given to peak current and average power dissipation so the
maximum specifications for the diode are not exceeded.
相關PDF資料 |
PDF描述 |
|---|---|
| ADP1147AN-3.3 | Current-Mode SMPS Controller |
| ADP1147AR-3.3 | Current-Mode SMPS Controller |
| ADP1147AN-33 | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| ADP1147AN-5 | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| ADP1147AR-33 | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| ADP1147-3.3 | 制造商:AD 制造商全稱:Analog Devices 功能描述:High Efficiency Step-Down Switching Regulator Controllers |
| ADP1147-5 | 制造商:AD 制造商全稱:Analog Devices 功能描述:High Efficiency Step-Down Switching Regulator Controllers |
| ADP1147AN-3.3 | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
| ADP1147AN-33 | 制造商:AD 制造商全稱:Analog Devices 功能描述:High Efficiency Step-Down Switching Regulator Controllers |
| ADP1147AN-5 | 制造商:Rochester Electronics LLC 功能描述:- Bulk 制造商:Analog Devices 功能描述:High Efficiency Step-Down Switching Regulator Controllers 制造商:Analog Devices Inc. 功能描述:High Efficiency Step-Down Switching Regulator Controllers |
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