NSR10F40QNXT5G Datasheet, PDF (4/6 Pages) ON Semiconductor – Schottky Diode Optimized

NSR10F40QNXT5G
APPLICATION SECTION
Introduction
As wireless devices become smaller and thinner more
compact, energy efficient, solutions are necessary. To
reduce the solution size many people will integrate various
discrete devices into the IC. While this may physically
reduce the part count this has some adverse side effects, such
as performance degradation. The best way to improve the
solution is to use optimized discrete devices that have been
shrunk and electrically optimized. In this paper we will
discuss the intricacies of choosing an optimized Schottky
diode for wireless devices.
First a discussion of high frequency boost converters as an
application is explored. Then various trends in space saving
and energy saving design will be discussed. Finally a stress
test and a bench tests are shown.
Background – Application
Most mobile phones use white LEDs to backlight the LCD
display. These white LEDs typically have a forward voltage
near 3.6 V. Since the typical power source in a mobile phone
is a single−cell Li−Ion battery that has an input voltage range
of 2.7 V to 4.2 V. Since more than one LED is required to
backlight a LCD panel either a single string (~up to 10 LEDs
in series) or multiple strings of LEDs (~ up to 10 LEDs in
series) in parallel are used.
An example of a single string inductive boost circuit is
shown in Figure 5. Typically, a very small voltage is
measured over a precision resistor in series with the LEDs
to feedback the output operation condition to the controller.
Many of today’s controllers integrate the transistors and the
diode to save space.
Figure 5. Simplified Typical Single Channel
Converter
Space Saving Ideas
The real issue with integrating all of the devices into the
controller is that these power devices have an increased
junction temperature compared to the controller. This
increased junction temperature can lead to reliability issues
due to the limited thermal conductivity of I.C. packages.
Another method for shrinking the size of an inductive
boost application is to increase the switching frequency.
When the switching frequency is increased a lower value
inductors can be used to keep a constant inductor current
ripple. Lower value capacitors can be used because they
become re−charged more frequently.
Unfortunately the transistor and the diode still need to
carry the same average and peak currents. The LEDs for a
backlight are generally set between 20 – 150 mA. This
means that the transistor and diode need to conduct up to and
above 1 A of current. If every element shrinks with
exception to the diode and FET then all of this effort is for
nothing. ON Semiconductor’s High Frequency optimized
schottky diodes solve this problem.
Using ON Semiconductor’s Optimized Schottky
Diodes
To continue to reduce space requirements for a
non−integrated, inductive boost circuit, a diode and a
transistor with low power dissipation during operation in a
package with high thermal conductivity is necessary. With
the compact nature of wireless applications the space is very
constrained and there is no place for a large heat sink (so a
thermally efficient package is required).
Typically for a 1 A diode with a R_qJA = 86°C/W a SMA
package is used. The SMA package is 5.21 mm x 2.60 mm
x 2.10 mm (L x W x H). ON Semiconductor’s new
optimized Schottky diode line these packages have a R_qJA
= 85 C/W and are only 1.4 mm x 0.6 mm x 0.27 mm (L x W
x H). This means that the same power can be dissipated in
only 8% of the total space. Not only is there is a thermal
conductivity density advantage but there is also a
performance improvement with these new optimized
Schottky diodes.
Thermal Stress Testing Bench Results
Before being tested a set of NSR10F40QNXT5G Boost
Optimized Schottky diodes were characterized for forward
voltage and reverse current over temperature. Next these
diodes were placed in a “1 MHz” Boost converter, operating
at near 750 kHz.
To augment the electrical stress seen on the
ON Semiconductor Schottky Diodes an inductive boost
regulator was set up with the following criteria: Input
Voltage = 2.3 V, Output Voltage = 32 V, Output Load Current
= 150 mA, L1 = 10 mH. This will cause higher than normal
currents to conduct through the diode.
To further augment the stress seen by the Schottky diode
a thermal component to the test was added when the
Schottky diodes were mounted to external PCBs with only
a minimum footprint pad size. Twisted, shielded pair cable
with an inductance of less then 0.125 mH attached the diode
PCBs to the “1 MHz” Boost board. This additional
inductance is modeled in Figure 6 as Lapra1 and Lpara2 and
seen as ringing. These cables allowed for the diode to run
inside of an oven set to 85°C for 48 Hours.
After the 48−hour test was completed the diodes were
taken back to the characterization lab for a post condition
analysis. This analysis showed that there was no shift in any
of the parameters, forward voltage, reverse leakage current,
and capacitance.
The graphs below shown below demonstrate the Pre and
Post−Stress characterization graphs and how that there was
no change in the part performance.
http://onsemi.com
4