LTC3538 Datasheet, PDF (11/16 Pages) Linear Technology – 800mA Synchronous Buck-Boost DC/DC Converter

LTC3538
OPERATION
Since the output current is discontinuous in boost mode,
the ripple in this mode will generally be much larger than
the magnitude of the ripple in buck mode.
Minimizing solution size is usually a priority. Please be
aware that ceramic capacitors can exhibit a significant
reduction in effective capacitance when a bias is applied.
The capacitors exhibiting the highest reduction are those
packaged in the smallest case size.
Input Capacitor Selection
Since V_IN is the supply voltage for the IC it is recommended
to place at least a 4.7μF, low ESR ceramic bypass capaci-
tor close to V_IN and GND. It is also important to minimize
any stray resistance from the converter to the battery or
other power source.
Optional Schottky Diodes
Schottky diodes across the synchronous switches B and
D are not required, but do provide a lower drop during the
break-before-make time (typically 15ns), thus improving
efficiency. Use a surface mount Schottky diode such as an
MBRM120T3 or equivalent. Do not use ordinary rectifier
diodes since their slow recovery times will compromise
efficiency.
Table 2. Capacitor Vendor Information
SUPPLIER PHONE
FAX
WEB SITE
AVX
[phone] [phone] www.avxcorp.com
Sanyo [phone] [phone] www.sanyovideo.com
Taiyo
Yuden
[phone] [phone] www.t-yuden.com
TDK
[phone] [phone] www.component.tdk.com
Shutdown MOSFET Selection
A discrete external N-channel MOSFET, open-drain pull-
down device or other suitable means can be used to put
the part in shutdown by pulling V_C below 0.25V. Since
the error amplifier sources 13μA typically when active
and 1.5μA in shutdown, a relatively high resistance pull-
down device can be used to pull V_C below 0.25V. More
importantly, leakage and parasitic capacitance need to
be minimized. During start-up, 1.5μA is typically sourced
from V_C. The leakage of an external pull-down device and
compensation components tied to V_C, must therefore be
minimized to ensure proper start-up. Capacitance from
the pull-down device should also be minimized as it can
affect converter stability. An N-channel MOSFET such as
the FDV301N or similar is recommended if an external
discrete N-channel MOSFET is needed.
PCB Layout Considerations
The LTC3538 switches large currents at high frequencies.
Special care should be given to the PCB layout to ensure
stable, noise-free operation. Figure 3 depicts the recom-
mended PCB layout to be utilized for the LTC3538. A few
key guidelines follow:
1. All circulating current paths should be kept as short as
possible. This can be accomplished by keeping the routes
to all components (except the FB divider network) in
Figure 3 as short and as wide as possible. Capacitor ground
connections should via down to the ground plane in the
shortest route possible. The bypass capacitor on V_IN should
be placed as close to the IC as possible and should have
the shortest possible paths to ground.
2. The small signal ground pad (GND) should have a single
point connection to the power ground. A convenient way
to achieve this is to short this pin directly to the Exposed
Pad as shown in Figure 3.
3. The components in bold and their connections should
all be placed over a complete ground plane.
4. To prevent large circulating currents from disrupting
the output voltage sensing, the ground for the resistor
divider should be returned directly to the small signal
ground (GND) as shown.
5. Use of vias in the attach pad will enhance the thermal
environment of the converter especially if the vias extend
to a ground plane region on the exposed bottom surface
of the PCB.
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