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| 型號(hào)： | LX1668CDW-TR |
| 廠商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封裝： | ROHS COMPLIANT, PLASTIC, SOWB-20 |
| 文件頁(yè)數(shù)： | 4/15頁(yè) |
| 文件大?。?/td> | 392K |
| 代理商： | LX1668CDW-TR |
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PROGRAMMABLE MULTIPLE OUTPUT DC:DC CONTROLLER
LX1668
PRODUCT DA T ABOOK 1996/1997
Copyright 1999
Rev. 1.0a
12
P RODUCTION D ATA S HEET
APPLICA TION INFORMA TION
CURRENT LIMIT (continued)
There are several alternative types of sense resistor. The
surface-mount metal “staple” form of resistor has the advantage of
exposure to free air to dissipate heat and its value can be
controlled very tightly. Its main drawback, however, is cost. An
alternative is to construct the sense resistor using a copper PCB
trace. Although the resistance cannot be controlled as tightly, the
PCB trace is very low cost.
PCB Sense Resistor
A PCB sense resistor should be constructed as shown in Figure 8.
By attaching directly to the large pads for the capacitor and
inductor, heat is dissipated efficiently by the larger copper masses.
Connect the current sense lines as shown to avoid any errors.
Recommended sense resistor sizes are given in the following
table:
Copper
Desired Resistor
Dimensions (w x l)
Weight
Thickness
Value
mm
inches
2 oz/ft2
68m
2.5m
2.5 x 22
0.1 x 0.85
5m
2.5 x 43
0.1 x 1.7
TABLE 3 - PCB Sense Resistor Selection Guide
2.5m
Sense Resistor
100mil Wide, 850mil Long
2.5mm x 22mm (2 oz/ft2 copper)
Output
Capacitor Pad
Inductor
Sense Lines
FIGURE 8 — Sense Resistor Construction Diagram
FIGURE 9 — Current Sense Circuit
R
L
R
S
C
S
V
CS
Current
Sense
Comparator
Load
R
S2
Loss-Less Current Sensing Using Resistance of Inductor
Any inductor has a parasitic resistance (R
L) which causes a DC
voltage drop when current flows through the inductor. Figure 9
shows a sensor circuit comprising of a surface mount resistor, R
S,
and capacitor, C
S, in parallel with the inductor, eliminating the
current sense resistor.
The current flowing through the inductor is a triangle wave. If
the sensor components are selected such that:
L/R
L = RS * CS
R
L
L/R
SCS
|T(
j ω)|
ω
1/R
SCS
R
L/L
FIGURE 10 — Sensor Gain
V
trip
L/(R
SCS)
V
trip
R
L
The voltage across the capacitor will be equal to the current
flowing through the resistor, i.e.
V
CS = ILRL
Since V
CS
reflects the inductor current, by selecting the
appropriate R
S and CS, VCS can used to sense current.
Design Example
(Pentium II circuit, with a maximum static current of 14.2A)
The gain of the sensor can be characterized as:
The dc/static tripping current I
trip,S satisfies:
I
trip,S =
Select L/R
SCS ≤ RL to have higher dynamic tripping current than
the static one. The dynamic tripping current I
trip,d satisfies:
I
trip,d =
O
B
SO
LETE
PRO
D
UCT
NOT
RECOMMENDED
FOR
NEW
DESIGNS
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