lx1669 Microsemi Corporation, lx1669 Datasheet - Page 12

• Manufacturer Part Number: lx1669
• Description: Programmable Controller
• Manufacturer: Microsemi Corporation

LX1669
12
CURRENT LIMIT
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 7.
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.
table:
Loss-Less Current Sensing Using Resistance of Inductor
Any inductor has a parasitic resistance (R
voltage drop when current flows through the inductor. Figure 8
shows a sensor circuit comprising of a surface mount resistor, R
and capacitor, C
current sense resistor.
the sensor components are selected such that:
Copper
Weight
2 oz/ft
There are several alternative types of sense resistor. The
Recommended sense resistor sizes are given in the following
The current flowing through the inductor is a triangle wave. If
L/R
FIGURE 7 — Sense Resistor Construction Diagram
2
TABLE 3 - PCB Sense Resistor Selection Guide
Inductor
L
= R
Thickness
Copper
68µm
S
* C
(continued)
S,
S
in parallel with the inductor, eliminating the
Desired Resistor
Sense Lines
2.5m
Value
5m
2.5m Sense Resistor
100mil Wide, 850mil Long
2.5mm x 22mm (2 oz/ft
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
P
R O G R A M M A B L E
P
2.5 x 22
2.5 x 43
Output
Capacitor Pad
L
A P P L I C A T I O N I N F O R M A T I O N
Dimensions (w x l)
) which causes a DC
mm
R O D U C T I O N
2
copper)
0.1 x 0.85
0.1 x 1.7
inches
D C : D C C
S
,
D
flowing through the resistor, i.e.
appropriate R
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
the static one. The dynamic tripping current I
A T A
The voltage across the capacitor will be equal to the current
Since V
Select L/R
V
I
I
L/R
trip,S
trip,d
CS
= I
O N T R O L L E R
S
S
=
|T(j )|
=
CS
C
R
L
S
H E E T
R
S
C
L
S
L/(R
V
L
reflects the inductor current, by selecting the
FIGURE 8 — Current Sense Circuit
S
R
Comparator
and C
trip
V
L
trip
R
S
C
FIGURE 9 — Sensor Gain
L
1/R
S
to have higher dynamic tripping current than
Current
)
S
, V
Sense
R
S
C
CS
S
L
S
can used to sense current.
trip,S
R
L
/L
V
satisfies:
CS
R
L
R
C
S2
S
trip,d
satisfies:
Copyright © 1999
Rev. 1.0
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