LX1681CDM-TR MICROSEMI [Microsemi Corporation], LX1681CDM-TR Datasheet - Page 7

• Manufacturer Part Number: LX1681CDM-TR
• Description: Voltage-Mode PWM Controllers
• Manufacturer: MICROSEMI [Microsemi Corporation]

Copyright © 2000
Rev. 1.1b,2005-03-09
diode. The use of a MOSFET (synchronous rectification) will
result in higher efficiency, but at higher cost than using a Schottky
diode (non-synchronous). Power dissipated in the bottom
MOSFET will be:
dissipate 0.6 * 15 * [1 – 2/5] = 5.4W (compared to the 1.8 to 3.5W
dissipated by a MOSFET under the same conditions). This power
loss becomes much more significant at lower duty cycles. The use
of a dual Schottky diode in a single TO-220 package (e.g. the
MBR2535) helps improve thermal dissipation.
provide sufficient drive to the upper MOSFET. In many
applications with a 5V (V
In situations where only 5V is present, V
a bootstrap (charge pump) circuit, as shown in Figure 4 (Typical
Applications section). The capacitor (C
up from V
when the FET is turned on. This scheme provides a V
equal to 2 * V
This voltage will provide sufficient gate drive to the external
MOSFET in order to get a low R
bootstrap circuit in synchronous rectification mode is likely to
result in faster turn-on than in non-synchronous mode.
thermal design of the demonstration 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. In our VRM module, heatsink area was taken from internal
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.
FET SELECTION (continued)
Synchronous Rectification – Lower MOSFET
The lower pass element can be either a MOSFET or a Schottky
[IRL3303 or 1.76W for the IRL3102]
A typical Schottky diode, with a forward drop of 0.6V will
The LX1681/1682 needs a secondary supply voltage (V
A great deal of time and effort were spent optimizing the
Non-Synchronous Operation - Schottky Diode
Operation From A Single Power Supply
LAYOUT GUIDELINES - THERMAL DESIGN
TM
P
D
CC
=
CC
CC
via the Schottky diode (D
I
2
- V
×
planes which were actually split and connected
R
DS
DS
(D
(
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
ON
2
), or approximately 9.5V with V
CC
)
×
) and a 12V (V
[
1
−
Duty
DS(ON)
4
Cycle
) is alternatively charged
C1
2
), and then boosted up
can be generated using
. Note that using the
C1
A P P L I C A T I O N I N F O R M A T I O N
) supply are present.
]
=
. 3
Linfinity Microelectronics Division
51
W
C1
®
Microsemi
CC
voltage
C1
= 5V.
) to
in order to minimize resistance and hence power losses. It is also
recommended that, whenever possible, the ground, input and
output power signals should be on separate planes (PCB layers).
See Figure 2 – bold traces are power traces.
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.
eration should be given to the layout of traces that carry high
currents. The main paths to consider are:
with any layout or component selection issues. A Gerber file with
layout for the most popular devices is available upon request.
Evaluation boards are also available upon request. Please check
Linfinity's web site for further application notes.
All of these traces should be made as wide and thick as possible,
FIGURE 2 — Enabling Linear Regulator
As always, be sure to provide local capacitive decoupling close
To reduce power losses due to ohmic resistance, careful consid-
Please contact Linfinity’s Applications Engineers for assistance
General Notes
Power Traces
Layout Assistance
Input power from 5V supply to drain of top MOSFET.
Trace between top MOSFET and lower MOSFET or
Schottky diode.
Trace between lower MOSFET or Schottky diode and
ground.
Trace between source of top MOSFET and inductor and
load.
Voltage-Mode PWM Controllers
LX168x
GND
P
RODUCTION
5V Input
D
ATA
S
HEET
LX1681/1682
Output
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