lx1660 Microsemi Corporation, lx1660 Datasheet - Page 11

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

Copyright © 1998
Rev. 1.1
INPUT CAPACITOR
The input capacitor and the input inductor are to filter the
pulsating current generated by the buck converter to reduce
interference to other circuits connected to the same 5V rail. In
addition, the input capacitor provides local de-coupling of the
buck converter. The capacitor should be rated to handle the RMS
current requirement. The RMS current is:
maximum value, when d = 50%, I
output in the range of 2 to 3V, the required RMS current is very
close to 0.5I
the drain of the top MOSFET and the source of the bottom one to
avoid ringing due to the parasitic inductor being switched ON and
OFF. See capacitor C
of this data sheet.
TIMING CAPACITOR SELECTION
The frequency of operation of the LX1660 / 1661 is a function of
the duty cycle and OFF-time. The OFF-time is proportional to the
timing capacitor (connected to Pin 8, C
minimize frequency variations with duty cycle. The frequency is
constant, during steady-state operation, due to the modulation of
the OFF-time.
ing equation:
(recommended to be around 200kHz for optimal operation and
component selection).
be approximated as follows:
frequency of 183kHz at V
used, the capacitor value must be changed (the optimal timing
capacitor for 12V input will be in the range of 1000 - 1500pF).
CURRENT LIMIT
Current limiting occurs when a sensed voltage, proportional to
load current, exceeds the current-sense comparator threshold
value (90mV). The current can be sensed either by using a fixed
sense resistor in series with the inductor to cause a voltage drop
proportional to current, or by using a resistor and capacitor in
parallel with the inductor to sense the voltage drop across the
7/98
where I
A high-frequency (ceramic) capacitor should be placed across
The timing capacitor (C
where I
When using a 5V input voltage, the switching frequency (f
Choosing a 680pF timing capacitor will result in an operating
I
C
C
RMS
T
T
=
= 0.621 *
= I
DIS
L
f
is the inductor current and d is the duty cycle. The
L
L
S
(1 - V
is fixed at 200µA and f
.
(1.52 - 0.29 * V
d(1-d)
OUT
I
DIS
f
7
S
/ V
in the Product Highlight on the first page
OUT
IN
T
) should be selected using the follow-
) * I
= 2.8V. When a 12V power input is
OUT
DIS
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
RMS
)
S
= 0.5I
is the switching frequency
U S I N G T H E L X 1 6 6 0 / 6 1 D E V I C E S
T
P
), and is modulated to
A
L
R O D U C T I O N
. For 5V input and
D V A N C E D
S
) can
P W M C
D
CURRENT LIMIT
parasitic resistance of the inductor. One should include an RC
filter at the CS+ and CS- inputs, as shown in the Application
Information section, to eliminate jitter and noise.
and C1 can be in the 300-500pF range as a starting point. If a fine
trim or adjustment of the current trip level is required, C1 may be
shunted by a resistor. C1 will introduce a small delay into the
current limit trip point, which effectively raises the threshold.
Sense Resistor
The current sense resistor (R1) is selected according to the for-
mula:
and I
below.
A smaller sense resistor will result in lower heat dissipation (I²R)
and also a smaller output voltage droop at higher currents.
face-mount metal “staple” form of resistor has the advantage of
exposure to free air to dissipate heat and its value can be con-
trolled very tightly. Its main drawback, however, is cost. An alter-
native is to construct the sense resistor using a copper PCB trace.
Although the resistance cannot be controlled as tightly, the PCB
trace is very low cost.
PCB Sense Resistor
A PCB sense resistor should be constructed as shown in Figure
7. 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.
Load
Pentium-Class Processor (<10A)
Pentium II Class (>10A)
A T A
For most applications, the resistors R5, R6 can be set at 1k ,
Where V
There are several alternative types of sense resistor. The sur-
O N T R O L L E R
TRIP
R1 = V
TABLE 2 - Current Sense Resistor Selection Guide
FIGURE 7 — Sense Resistor Construction Diagram
is the desired current limit. Typical choices are shown
S
Inductor
TRIP
TRIP
H E E T
is the current sense comparator threshold (100mV)
/ I
(continued)
TRIP
Sense Lines
2.5m
100mil Wide, 850mil Long
2.5mm x 22mm (2 oz/ft
LX1660/1661
Sense Resistor
Sense Resistor Value
Output
Capacitor Pad
2.5m
5m
2
copper)
11