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| 型号： | LX1660CD-TR |
| 厂商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分类： | 稳压器 |
| 英文描述： | 1.5 A SWITCHING CONTROLLER, PDSO16 |
| 封装： | LEAD FREE, PLASTIC, SOIC-16 |
| 文件页数： | 4/15页 |
| 文件大小： | 376K |
| 代理商： | LX1660CD-TR |
A D V ANCED PWM C ONTROLLER
LX1660/1661
PRODUCT DA T ABOOK 1996/1997
Copyright 1998
Rev. 1.1a 10/25/2004
12
P RODUCTION D ATA S HEET
USING THE LX1660/61 DEVICES
CURRENT LIMIT (continued)
Recommended sense resistor sizes are given in the following
table:
CURRENT LIMIT (continued)
The dc/static tripping current I
trip,S satisfies:
I
trip,S =
Select L/R
SCS ≤ RL to have higher dynamic tripping curr ent
than the static one. The dynamic tripping current I
trip,d satisfies:
I
trip,d =
General Guidelines for Selecting R
S , CS , and RL
R
L =
Select: R
S ≤ 10 k
and C
S according to:
C
Sn =
The above equation has taken into account the current-de-
pendency of the inductance. Typical values are: R
L = 3m, RS =
9k
, C
S = 0.1F, and L is 2.5H at 0A current.
In cases where R
L is so large that the trip point current would
be lower than the desired short-circuit current limit, a resistor (R
S2)
can be put in parallel with C
S, as shown in Figure 8. The selection
of components is as follows:
=
C
S =
=
*
Again, select (R
S2 //RS) < 10k .
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 = C
T , where
C
T is the timing capacitor.
Refer to Capacitor C4 in the Applica-
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
HICCUP by using:
Duration of reduced D operation = 100ms/F
R
L
L/R
SCS
|T(j
ω)|
ω
1/R
SCS
R
L/L
FIGURE 9 — Sensor Gain
FIGURE 8 — Current Sense Circuit
R
L
R
S
C
S
V
CS
Current
Sense
Comparator
Load
R
S2
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
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 8
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 *
C
S
The voltage across the capacitor will be equal to the current
flowing through the resistor, i.e.
V
CS = I LRL
Since V
CS reflects the inductor current, by selecting the appro-
priate R
S
and C
S , VCS can be made to reach the comparator
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:
V
trip
L/(R
SCS)
V
trip
R
L
V
trip
I
trip,S
L
n
R
L RS
R
L (Required)
R
L (Actual)
R
S2
R
S2 + RS
L
R
L (Actual) * (RS2 // RS)
L
R
L (Actual)
R
S + RS2
R
S2 * RS
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