lx1660 Microsemi Corporation, lx1660 Datasheet - Page 12

• Manufacturer Part Number: lx1660
• Description: Advanced Pwm Controller
• Manufacturer: Microsemi Corporation

LX1660/1661
12
CURRENT LIMIT
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.
If the sensor components are selected such that:
flowing through the resistor, i.e.
priate R
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:
Copper
Weight
2 oz/ft
Recommended sense resistor sizes are given in the following
The current flowing through the inductor is a triangle wave.
The voltage across the capacitor will be equal to the current
Since V
L/R
V
CS
2
TABLE 3 - PCB Sense Resistor Selection Guide
S
L
= I
L/R
= R
CS
and C
Thickness
L
Copper
reflects the inductor current, by selecting the appro-
R
S
68µm
|T(j )|
S
C
R
L
FIGURE 8 — Current Sense Circuit
* C
S
L
Comparator
(continued)
S,
S
, V
S
in parallel with the inductor, eliminating the
FIGURE 9 — Sensor Gain
1/R
Current
CS
Desired Resistor
Sense
can be made to reach the comparator
S
R
C
S
L
S
2.5m
Value
5m
R
L
/L
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
V
CS
R
L
U S I N G T H E L X 1 6 6 0 / 6 1 D E V I C E S
R
C
S2
S
P
2.5 x 22
2.5 x 43
L
A
Dimensions (w x l)
, which causes a DC
mm
R O D U C T I O N
D V A N C E D
0.1 x 0.85
0.1 x 1.7
inches
Load
P W M C
S
,
D
CURRENT LIMIT
The dc/static tripping current I
than the static one. The dynamic tripping current I
General Guidelines for Selecting R
pendency of the inductance. Typical values are: R
9k , C
be lower than the desired short-circuit current limit, a resistor (R
can be put in parallel with C
of components is as follows:
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 C
C
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
A T A
T
Select L/R
and C
The above equation has taken into account the current-de-
In cases where R
Again, select (R
is the timing capacitor. Refer to Capacitor C4 in the Applica-
Duration of reduced D operation = 100ms/µF
O N T R O L L E R
I
I
R
C
trip,S
trip,d
S
L
R
S
R
=
= 0.1µF, and L is 2.5µH at 0A current.
L (Required)
=
S
L (Actual)
S
=
=
according to:
R
I
V
trip,S
L (Actual)
H E E T
S
L/(R
trip
C
V
R
S
trip
V
L
(continued)
trip
S
C
S2
=
R
S
L
* (R
)
//R
L
L
is so large that the trip point current would
R
to have higher dynamic tripping current
S2
S
) < 10k .
S2
R
+ R
S2
// R
HICCUP
S
Select: R
C
, as shown in Figure 8. The selection
S
S n
S
)
trip,S
=
by using:
=
satisfies:
R
S
L
L
R
S
, C
n
R
L (Actual)
S
S
L
10 k
, and R
*
L
R
R
L
S
S2
trip,d
= 3m , R
+ R
* R
Copyright © 1998
satisfies:
T
Rev. 1.1
S2
S
, where
S
S2
=
7/98
)