lx1684 Microsemi Corporation, lx1684 Datasheet - Page 8

• Manufacturer Part Number: lx1684
• Description: Voltage-mode Pwm Controller
• Manufacturer: Microsemi Corporation

Copyright © 2000
Rev. 1.0c, 2005-03-08
FET SELECTION
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 – 3.3/12] = 6.5W (compared to the 2.1 to
4.2W 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.
sufficient drive to the upper MOSFET. The top FET must be a
logic level power MOSFET such as
to turn on to a low R
generated using a bootstrap (charge pump) circuit, as shown in the
Product Highlight on page 1. The capacitor, (C
charged up from 5V via the Schottky diode, (D
up when the FET is turned on. Under any circumstance the voltage
at V
not be greater than 19V for more than 50nS. Lab evaluation and
module production test should be the final arbiter to verify the
proper operation. For application with a large MOSFET, the
maximum voltage at V
dissipation in the FET driver section. It is inherent in a higher
current power supply that the parasitic inductance and capacitance
on PCB board and Power MOSFET device induces ringing at the
gate drive. The extra thermal dissipation and the higher peak
voltage generated by gate ringing should be taken in account
during final design. The temperature rise due to gate drive thermal
dissipation can be reduced by extra heat sinking. A resistor in
series with the gate in order of 10ohm or snubber circuitry can
reduce the gate ringing. The voltage must provide sufficient gate
drive to the external MOSFET in order to get a low R
MUST be lower than maximum voltage rating of 18V.
mode is likely to result in faster turn-on than in non-synchronous
mode.
LAYOUT GUIDELINES - THERMAL DESIGN
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.
The lower pass element can be either a MOSFET or a Schottky
[SUB45N03-13L or 2.12W for the SUB70N03-09BP]
A typical Schottky diode, with a forward drop of 0.6V will
The LX1684 needs a secondary supply voltage (V
Note that using the bootstrap circuit in synchronous rectification
A great deal of time and effort were spent optimizing the
Synchronous Rectification – Lower MOSFET
Non-Synchronous Operation - Schottky Diode
Boost Operation
C1
should not be more than 18V for more than 300nS and must
TM
P
D
=
I
2
×
R
(continued)
DS
DS(ON)
(
ON
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
C1
)
with V
×
should be kept lower due to thermal
[
1
−
Duty
GS
SUB45N03-13L
of 4.5V or higher. V
Cycle
A P P L I C A T I O N I N F O R M A T I O N
]
1
), and then boosted
=
1
) is alternatively
3
. It must be able
.
26
Integrated Products Division
C1
) to provide
W
DS(ON)
®
C1
Microsemi
can be
and
If the FET switches have been carefully selected, external
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:
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..
All of these traces should be made as wide and thick as possible,
As always, be sure to provide local capacitive decoupling close
To reduce power losses due to ohmic resistance, careful consid-
General Notes
Power Traces
Input power from 12V 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 Controller
LX1684
GND
FIGURE 2 — Key Power PCB Traces
P
RODUCTION
+12V Input
D
ATA
S
HEET
Output
LX1684
Page 8