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LX1660 Datasheet PDF : 15 Pages
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PRODUCT DATABOOK 1996/1997
LX1660/1661
ADVANCED PWM CONTROLLER
PRODUCTION DATA SHEET
USING THE LX1660/61 DEVICES
CURRENT LIMIT (continued)
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
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
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 8
shows a sensor circuit comprising of a surface mount resistor, RS,
and capacitor, CS, in parallel with the inductor, eliminating the
current sense resistor.
L
RL
Load
RS
CS
Current
Sense
Comparator
VCS RS2
FIGURE 8 — Current Sense Circuit
The current flowing through the inductor is a triangle wave.
If the sensor components are selected such that:
L/RL = RS * CS
The voltage across the capacitor will be equal to the current
flowing through the resistor, i.e.
VCS = ILRL
Since V reflects the inductor current, by selecting the appro-
CS
priate R and C , V can be made to reach the comparator
S
S
CS
voltage at the desired trip current.
Design Example
(Pentium II circuit, with a maximum static current of 14.2A)
The gain of the sensor can be characterized as:
|T(jω)|
RL
L/RSCS
CURRENT LIMIT (continued)
The dc/static tripping current I satisfies:
trip,S
Itrip,S =
Vtrip
RL
Select L/R C ≤ R to have higher dynamic tripping current
SS
L
than the static one. The dynamic tripping current Itrip,d satisfies:
I=
trip,d
Vtrip
L/(RSCS)
General Guidelines for Selecting RS , CS , and RL
R=
L
V
trip
I
trip,S
and CS according to:
Select: R ≤ 10 kΩ
S
L
CS n =
n
RR
LS
The above equation has taken into account the current-de-
pendency of the inductance. Typical values are: RL = 3mΩ, RS =
9kΩ, C = 0.1µF, and L is 2.5µH at 0A current.
S
In cases where RL is so large that the trip point current would
be lower than the desired short-circuit current limit, a resistor (RS2)
can be put in parallel with C , as shown in Figure 8. The selection
S
of components is as follows:
RL (Required)
RL (Actual)
=
RS2
RS2 + RS
L
L
R +R
C=
=
S
R
L
(Actual)
*
(R
S2
//
R)
S
R
*
L (Actual)
S
S2
R
S2
*
R
S
Again, select (R //R ) < 10kΩ.
S2 S
C4 ERROR COMPARATOR INPUT BYPASS CAPACITOR
The LX1660/61 device has a unique topology which results in
extremely fast response to transient disturbances. Actual loop
closure is around a comparator. A capacitor should be placed
between the INV and NINV Error Comparator inputs to eliminate
jitter and noise. This capacitor value should be: C = ½ CT, where
C is the timing capacitor. Refer to Capacitor C4 in the Applica-
T
tion Information section.
C7 HICCUP CAPACITOR SELECTION
The hiccup capacitor controls two time periods; the ON time
duration of 10% duty cycle mode, and the OFF-time duration
before re-try. The ON:OFF-time ratios will always be 1:10 due to
the current sources which charge (10I) and discharge (I) the
hiccup capacitor. Select C by using:
HICCUP
ω
1/RSCS RL/L
FIGURE 9 — Sensor Gain
Duration of reduced D operation = 100ms/µF
12
Copyright © 1998
Rev. 1.1 7/98
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