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參數(shù)資料
| 型號： | EL7571C |
| 英文描述： | Programmable PWM Controller |
| 中文描述： | 可編程PWM控制器 |
| 文件頁數(shù)： | 2/23頁 |
| 文件大小： | 480K |
| 代理商： | EL7571C |
10
EL7571C
Programmable PWM Controller
E
L
75
71
C
There are two criteria for selecting the resistor value and
type. Firstly, the minimum value is limited by the maxi-
mum output current. The EL7571C current limit
capacitor has a typical threshold of 154mV, 125mV
minimum. When the voltage across the sense resistor
exceeds this threshold, the conduction cycle of the top
switch terminates immediately, providing pulse by pulse
current limiting. A resistor value must be selected which
guarantees operation under maximum load. That is:
where:
VOCMIN = minimum over current voltage threshold
IMAX = maximum output current
Secondly, since the load current passes directly through
the sense resistor, its power rating must be sufficient to
handle the power dissipated during maximum load (cur-
rent limit) conditions. Thus:
where:
PD = power dissipated in current sense resistor
PD must be less than the power rating of the current
sense resistor. High current applications may require
parallel sense resistors to dissipate sufficient power.
Current Sense Resistor Table below lists some popular
current sense resistors: the WLS-2512 series of Power
Metal Strip Resistors from Dale Electronics, OARS
series Iron Alloy resistor from IRC, and Copper Magna-
nin (CuNi) wire resistor from Mills Resistors. Mother
board copper trace is not recommended because of its
high temperature coefficient and low power dissipation.
The trade-off between the different types of resistors are
cost, space, packaging and performance. Although
Power Metal Strip Resistors are relatively expensive,
they are available in surface mount packaging with
tighter tolerances. Consequently, less board space is
used to achieve a more accurate current sense. Alterna-
tively, Magnanin copper wire has looser tolerance and
higher parasitic inductance. This results in a less current
sense but at a much lower cost. Metal track on the PCB
can also be used as current sense resistor. The trade-offs
are ±30% tolerance and ±4000 ppm temperature coeffi-
cient. Ultimately, the selection of the type of current
sense element must be made on an application by appli-
cation basis.
Input Capacitor, C1
In a buck converter, where the output current is greater
than 10A, significant demand is placed on the input
capacitor. Under steady state operation, the high side
FET conducts only when it is switched “on” and con-
ducts zero current when it is turned “off”. The result is a
current square wave drawn from the input supply. Most
of this input ripple current is supplied from the input
capacitor C1. The current flow through C1’s equivalent
series resistance (ESR) can heat up the capacitor and
cause premature failure. Maximum input ripple current
occurs when the duty cycle is 50%, a current of Iout/2
RMS.
Worst case power dissipation is:
where:
ERSIN = input capacitor ESR
R1
VOCMIN
1MAX
---------------------
=
PD
1OUTMAX
2
R1
×
=
Bill of Materials
Manufacturer
Part No.
Tolerance
Temperature
Coefficient
Power Rating
Phone No.
Fax No.
Dale
WSL 2512
±1%
±75ppm
1 W
402-563-6506
402-563-6418
IRC
OARS Series
±5%
±20ppm
1W - 5W
800-472-6467
800-472-3282
Mills Resistor
MRS1367-TBA
±10%
±20ppm
1.2W
916-422-5461
906-422-1409
PCB Trace Resistor
±30%
±4000ppm
50A/in (1oz Cu)
PD
IOUT
2
-------------
2
ESRIN
=
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