lx1665 Microsemi Corporation, lx1665 Datasheet - Page 13

• Manufacturer Part Number: lx1665
• Description: Dual Output Pwm Controllers With 5-bit Dac
• Manufacturer: Microsemi Corporation

CURRENT LIMIT
PCB Sense Resistor
10. 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.
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 11
shows a sensor circuit comprising of a surface mount resistor, R
and capacitor, C
current sense resistor.
A PCB sense resistor should be constructed as shown in Figure
Recommended sense resistor sizes are given in the following
FIGURE 10 — Sense Resistor Construction Diagram
TABLE 3 - PCB Sense Resistor Selection Guide
Inductor
FIGURE 11 — Current Sense Circuit
Comparator
(continued)
S,
in parallel with the inductor, eliminating the
Current
Sense
R
Sense Lines
L
S
2.5m9
100mil Wide, 850mil Long
2.5mm x 22mm (2 oz/ft
Ω
Ω
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
D
V
CS
UAL
Sense Resistor
R
U S I N G T H E L X 1 6 6 4 / 6 5 D E V I C E S
L
R
C
O
P
Output
Capacitor Pad
S2
S
L
UTPUT
, which causes a DC
R O D U C T I O N
2
PWM C
copper)
Load
ONTROLLERS WITH
S
,
LX1664/1664A, LX1665/65A
D
CURRENT LIMIT
The current flowing through the inductor is a triangle wave. If the
sensor components are selected such that:
flowing through the resistor, i.e.
appropriate R
voltage (60mV for LX166xA or 100mV for the LX166x) 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:
The dc/static tripping current I
than the static one. The dynamic tripping current I
General Guidelines for Selecting R
pendency of the inductance.
R
A T A
L
= 3mΩ, R
The voltage across the capacitor will be equal to the current
Since V
Select L/R
and C
The above equation has taken into account the current-de-
The test circuit (Figure 6) used the following parameters:
L/R
V
I
I
R
trip,S
trip,d
L
CS
L/R
=
S
L
= I
S
= R
according to:
=
=
CS
S
I
V
|T(j M )|
L
S
C
trip,S
H E E T
R
R
S
trip
S
S
L/(R
C
= 9kΩ, C
S
L
V
reflects the inductor current, by selecting the
L
5-B
* C
R
and C
S
trip
V
(continued)
L
≤ R
trip
S
FIGURE 12 — Sensor Gain
C
S
IT
1/R
S
)
L
S
, V
to have higher dynamic tripping current
DAC
S
S
C
= 0.1µF, and L is 2.5µH at 0A current.
CS
S
can be made to reach the comparator
Select: R
C
S n
R
trip,S
L
/L
=
satisfies:
R
S
L
L
S
, C
n
R
≤ 10 kΩ
S
S
, and R
L
M
trip,d
satisfies:
13