LX1668 Ver la hoja de datos (PDF) - Microsemi Corporation
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LX1668 Datasheet PDF : 15 Pages
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PRODUCT DATABOOK 1996/1997
LX1668
P ROGRAMMABLE MULTIPLE OUTPUT DC:DC CONTROLLER
PRODUCTION DATA SHEET
APPLICATION INFORMATION
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.
Inductor
2.5mΩ Sense Resistor
100mil Wide, 850mil Long
2.5mm x 22mm (2 oz/ft2 copper)
L
RL
Load
RS
Current
Sense
Comparator
CS
RS2
VCS
FIGURE 9 — Current Sense Circuit
The voltage across the capacitor will be equal to the current
flowing through the resistor, i.e.
VCS = ILRL
Since VCS reflects the inductor current, by selecting the
appropriate RS and CS, VCS can used to sense current.
Output
Capacitor Pad
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 Itrip,S satisfies:
Sense Lines
|T(j ω)|
FIGURE 8 — Sense Resistor Construction Diagram
Recommended sense resistor sizes are given in the following
table:
TABLE 3 - PCB Sense Resistor Selection Guide
Copper Copper Desired Resistor
Weight Thickness
Value
Dimensions (w x l)
mm
inches
RL
L/RSCS
2 oz/ft2 68µm
2.5mΩ
5mΩ
2.5 x 22
2.5 x 43
0.1 x 0.85
0.1 x 1.7
ω
1/RSCS RL/L
Loss-Less Current Sensing Using Resistance of Inductor
Any inductor has a parasitic resistance (RL) which causes a DC
voltage drop when current flows through the inductor. Figure 9
shows a sensor circuit comprising of a surface mount resistor, RS,
and capacitor, CS, 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/RL = RS * CS
FIGURE 10 — Sensor Gain
Itrip,S =
Vtrip
RL
Select L/RSCS ≤ RL to have higher dynamic tripping current than
the static one. The dynamic tripping current Itrip,d satisfies:
V
Itrip,d =
trip
L/(R C )
SS
12
Copyright © 1999
Rev. 1.0 4/99
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