LTC3421_15 Datasheet, PDF (12/16 Pages) Linear Technology – 3A, 3MHz Micropower Synchronous Boost Converter with Output Disconnect

LTC3421
APPLICATIO S I FOR ATIO
Output Capacitor Selection
The output voltage ripple has two components to it. The
bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The maximum
ripple due to charge is given by:
V_RBULK
=
I_P • V_IN
C_OUT • V_OUT
•
f
where I_P = peak inductor current.
The ESR (equivalent series resistance) is usually the most
dominant factor for ripple in most power converters. The
ripple due to capacitor ESR is simply given by:
V_RCESR = I_P • C_ESR
where C_ESR = capacitor series resistance.
Low ESR capacitors should be used to minimize output
voltage ripple. For surface mount applications, AVX TPS
series tantalum capacitors, Sanyo POSCAP or Taiyo Yuden
ceramic capacitors are recommended. For through-hole
applications, Sanyo OS-CON capacitors offer low ESR in a
small package size.
In some layouts it may be necessary to place a 1µF low ESR
ceramic capacitor as close to the V_OUT and GND pins as
possible.
Input Capacitor Selection
The input filter capacitor reduces peak currents drawn
from the input source and reduces input switching noise.
Since the IC can operate at voltages below 0.5V once the
output is regulated, the demand on the input capacitor is
much less. In most applications 1µF per amp of peak input
current is recommended. Taiyo Yuden offers very low ESR
ceramic capacitors, for example the 1µF in a 0603 case
(JMK107BJ105MA).
Table 2. Capacitor Vendor Information
SUPPLIER PHONE
FAX
WEB SITE
AVX
[phone] [phone] www.avxcorp.com
Sanyo
[phone] [phone] www.sanyovideo.com
TDK
[phone] [phone] www.component.tdk.com
Murata
USA:
USA:
www.murata.com
[phone] [phone]
[phone]
Taiyo Yuden [phone] [phone] www.t-yuden.com
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Operating Frequency Selection
There are several considerations in selecting the operating
frequency of the converter. The first is, which are the sen-
sitive frequency bands that cannot tolerate any spectral
noise? The second consideration is the physical size of the
converter. As the operating frequency goes up, the induc-
tor 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.
Another operating frequency consideration is whether the
application can allow “pulse skipping.” In this mode, the
minimum on time of the converter cannot support the duty
cycle, so the converter ripple will go up and there will be
a low frequency component of the output ripple. In many
applications where physical size is the main criterion,
running the converter in this mode is acceptable. In appli-
cations where it is preferred not to enter this mode, the
maximum operating frequency is given by:
f_{MAX _NOSKIP}
=
V_OUT – V_IN
V_OUT • t_ON(MIN)
Hz
where t_ON(MIN) = minimum on time = 120ns.
Thermal Considerations
To deliver the power that the LTC3421 is capable of, it is
imperative that a good thermal path be provided to
dissipate the heat generated within the package. This can
be accomplished by taking advantage of the large thermal
pad on the underside of the IC. It is recommended that mul-
tiple vias in the printed circuit board be used to conduct
heat away from the IC and into a copper plane with as much
area as possible. In the event that the junction tempera-
ture gets too high, the peak current limit will automatically
be decreased. If the junction temperature continues to rise,
the part will go into thermal shutdown, and all switching
will stop until the temperature drops.
V_IN > V_OUT Operation
The LTC3421 will maintain voltage regulation when the
input voltage is above the output voltage. This is achieved
by terminating the switching on the synchronous PMOS
and applying V_IN statically on the gate. This will ensure the
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