EL7571C Ver la hoja de datos (PDF) - Elantec -> Intersil
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EL7571C Datasheet PDF : 23 Pages
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EL7571C
Programmable PWM Controller
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:
R1 = -V---1O---M--C---AM----X-I--N--
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:
PD = 1OUTMAX2 × R1
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.
Bill of Materials
Manufacturer
Dale
IRC
Mills Resistor
PCB Trace Resistor
Part No.
WSL 2512
OARS Series
MRS1367-TBA
Tolerance
±1%
±5%
±10%
±30%
Temperature
Coefficient
±75ppm
±20ppm
±20ppm
±4000ppm
Power Rating
1W
1W - 5W
1.2W
50A/in (1oz Cu)
Phone No.
402-563-6506
800-472-6467
916-422-5461
Fax No.
402-563-6418
800-472-3282
906-422-1409
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:
PD=
I---O---2-U----T-
2
•
E
S
RIN
where:
ERSIN = input capacitor ESR
10
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