P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1664/1664A, LX1665/65A
DUAL OUTPUT PWM CONTROLLERS WITH 5-BIT DAC
P R O D U C T I O N D A T A S H E E T
USING THE LX1664/65 DEVICES
CURRENT LIMIT (continued)
CURRENT LIMIT (continued)
The current flowing through the inductor is a triangle wave. If the
sensor components are selected such that:
PCB Sense Resistor
A PCB sense resistor should be constructed as shown in Figure
10. By attaching directly to the large pads for the capacitor and
inductor, heatisdissipatedefficientlybythelargercoppermasses.
Connect the current sense lines as shown to avoid any errors.
L/RL = RS CS
*
The voltage across the capacitor will be equal to the current
flowing through the resistor, i.e.
2.5m
W
Sense Resistor
VCS = ILRL
100mil Wide, 850mil Long
2.5mm x 22mm (2 oz/ft2 copper)
Since VCS reflects the inductor current, by selecting the
appropriate RS and CS, VCS can be made to reach the comparator
voltage (60mV for LX166xA or 100mV for the LX166x) at the
desired trip current.
Inductor
Design Example
(Pentium II circuit, with a maximum static current of 14.2A)
The gain of the sensor can be characterized as:
Output
Capacitor Pad
|T( )|
j
w
Sense Lines
FIGURE 10 — Sense Resistor Construction Diagram
RL
Recommended sense resistor sizes are given in the following
table:
L/RSCS
TABLE 3 - PCB Sense Resistor Selection Guide
Copper Copper Desired Resistor Dimensions (w x l)
Weight Thickness Value
2 oz/ft2
68µm 2.5m
5m
w
mm
inches
1/RSCS RL/L
FIGURE 12 — Sensor Gain
Ω
2.5 x 22
2.5 x 43
0.1 x 0.85
0.1 x 1.7
Ω
The dc/static tripping current Itrip,S satisfies:
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 11
shows a sensor circuit comprising of a surface mount resistor, RS,
and capacitor, CS, in parallel with the inductor, eliminating the
current sense resistor.
Vtrip
Itrip,S
=
RL
Select L/RSCS ≤ RL to have higher dynamic tripping current
than the static one. The dynamic tripping current Itrip,d satisfies:
Vtrip
Itrip,d
=
L/(RSCS)
L
RL
Load
General Guidelines for Selecting RS , CS , and RL
Vtrip
RL =
Select: RS ≤ 10 kΩ
Itrip,S
RS
Ln
CS
RS2
and CS according to:
CS n
=
RL RS
Current
Sense
Comparator
The above equation has taken into account the current-de-
pendency of the inductance.
VCS
The test circuit (Figure 6) used the following parameters:
RL = 3mΩ, RS = 9kΩ, CS = 0.1µF, and L is 2.5µH at 0A current.
FIGURE 11 — Current Sense Circuit
Copyright © 1999
Rev. 1.2 11/99
13
MICROSEMI [ Microsemi ]
