LTC3440
13
3440fd
For more information www.linear.com/LTC3440
applications inForMation
Input Voltage > 4.5V
For applications with input voltages above 4.5V which could
exhibit an overload or short-circuit condition, a 2Ω/1nF
series snubber is required between the SW1 pin and GND.
A Schottky diode such as the Phillips PMEG2010EA or
equivalent from SW1 to V
IN
should also be added as close
to the pins as possible. For the higher input voltages V
IN
bypassing becomes more critical, therefore, a ceramic
bypass capacitor as close to the V
IN
and GND pins as
possible is also required.
Operating Frequency Selection
There are several considerations in selecting the operat
-
ing frequency of the converter. The first is, what are the
sensitive frequency bands that cannot tolerate any spec-
tral noise? For example, in products incorporating RF
communications, the
455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz and in
that case a 2MHz converter frequency may be employed.
Other considerations are the physical size of the converter
and efficiency. As the operating frequency goes up, the
inductor and filter capacitors go down in value and size.
The trade off is in efficiency since the switching losses due
to gate charge are going up proportional with frequency.
Additional quiescent current due to the output switches
GATE charge is given by:
Buck: (0.5 • V
IN
• f) mA
Boost: [0.25 • (V
IN
+ V
OUT
) • f ] mA
Buck/Boost: f • (0.75 • V
IN
+ 0.25 • V
OUT
) mA
where f = switching frequency in MHz
The other component of ripple is due to the ESR (equiva
-
lent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden ceramic
capacitors, AVX TPS series tantalum capacitors or Sanyo
POSCAP are recommended.
Input Capacitor Selection
Since the V
IN
pin is the supply voltage for the IC it is
recommended to place at least a 4.7µF, low ESR bypass
capacitor.
Table 2. Capacitor Vendor Information
SUPPLIER PHONE FAX WEB SITE
AVX (803) 448-9411 (803) 448-1943 www.avxcorp.com
Sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com
Taiyo Yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com
Optional Schottky Diodes
To achieve a 1%-2% efficiency improvement above
50mW, Schottky diodes can be added across synchronous
switches B (SW1 to GND) and D (SW2 to V
OUT
). The
Schottky diodes will provide a lower voltage drop during
the break-before-make time (typically 15ns) of the NMOS to
PMOS transition. General purpose diodes such as a 1N914
are not recommended due to the slow recovery times and
will compromise efficiency. If desired a large Schottky
diode, such as an MBRM120T3, can be used from SW2 to
V
OUT
. A low capacitance Schottky diode is recommended
from GND to SW1 such as a Phillips PMEG2010EA or
equivalent.
Output Voltage > 4.3V
A Schottky diode from SW to V
OUT
is required for output
voltages over 4.3V. The diode must be located as close to
the pins as possible in order to reduce the peak voltage
on SW2 due to the parasitic lead and trace inductance.
