lx1665 Microsemi Corporation, lx1665 Datasheet - Page 16

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

LX1664/64A, LX1665/65A
16
LINEAR REGULATOR
Capacitor Selection
Referring to the Product Highlight schematic on the front page, the
standard value to use as the linear regulator stage output capacitor
is on the order of 330µF. This provides sufficient hold-up for all
expected transient load events in memory and I/O circuitry.
Disabling Linear Output
Linear regulator output can be disabled by pulling feedback pin
(L
FB
Resistors Settings for Linear Regulator Output Voltage
) up to 5V as shown in Figure 14.
LIN EN
TABLE 7 - Linear Enable (LIN EN) Function Table
FIGURE 14 — Enabling Linear Regulator
LX1664
L
DRV
L
10k
FB
LIN EN
10
9
(continued)
H
L
2N2222
5V
10k
Ω Ω Ω Ω Ω
TABLE 6 -
0.1µF
C
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
10
D
LIN OUTPUT
UAL
R
R
Disabled
∞
Enabled
5
6
U S I N G T H E L X 1 6 6 4 / 6 5 D E V I C E S
Ω Ω Ω Ω Ω
Q
IRLZ44
O
P
4
UTPUT
R O D U C T I O N
C
330µF
9
C
330µF
7
Supply Voltage
For I/O Chipset
PWM C
ONTROLLERS WITH
D
LAYOUT GUIDELINES - THERMAL DESIGN
A great deal of time and effort were spent optimizing the thermal
design of the demo boards. Any user who intends to implement
an embedded motherboard would be well advised to carefully
read and follow these guidelines. If the FET switches have been
carefully selected, external heatsinking is generally not required.
However, this means that copper trace on the PC board must now
be used. This is a potential trouble spot; as much copper area as
possible must be dedicated to heatsinking the FET switches, and
the diode as well if a non-synchronous solution is used.
ground and V
with VIAS to the power device tabs. The TO-220 and TO-263
cases are well suited for this application, and are the preferred
packages. Remember to remove any conformal coating from all
exposed PC traces which are involved in heatsinking.
General Notes
As always, be sure to provide local capacitive decoupling close to
the chip. Be sure use ground plane construction for all high-
frequency work. Use low ESR capacitors where justified, but be
alert for damping and ringing problems. High-frequency designs
demand careful routing and layout, and may require several
iterations to achieve desired performance levels.
Power Traces
To reduce power losses due to ohmic resistance, careful consid-
eration should be given to the layout of traces that carry high
currents. The main paths to consider are:
A T A
In our VRM module, heatsink area was taken from internal
I Input power from 5V supply to drain of top MOSFET.
I Trace between top MOSFET and lower MOSFET or Schottky
I Trace between lower MOSFET or Schottky diode and
I Trace between source of top MOSFET and inductor, sense
diode.
ground.
resistor and load.
LX166x
S
H E E T
CC
5-B
planes which were actually split and connected
FIGURE 15 — Power Traces
IT
DAC
5V or 12V
Input
Output