LTC3533 Datasheet, PDF (12/16 Pages) Linear Technology – 2A Wide Input Voltage Synchronous Buck-Boost DC/DC Converter

LTC3533
APPLICATIONS INFORMATION
requirements of the converter. As a rule of thumb, the ratio
of the operating frequency to the unity-gain bandwidth of
the converter is the amount the output capacitance will
have to increase from the above calculations in order to
maintain the desired transient response.
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 or
TDK ceramic capacitors, AVX TPS series tantalum capaci-
tors or Sanyo POSCAP are recommended. See Table 2 for
contact information.
Input Capacitor Selection
Since PV_IN is the supply voltage for the IC it is recom-
mended to place at least a 4.7µF, low ESR ceramic bypass
capacitor close to PV_IN and GND. It is also important to
minimize any stray resistance from the converter to the
battery or other power source.
to provide the conduction path to the output. Note that
Burst Mode operation is inhibited at output voltages below
1V typical.
Output Voltage > 4.3V
A Schottky diode from SW2 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 parasitic lead and trace inductances.
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 SW1 and
GND. A Schottky diode from SW1 to PV_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 PV_IN and GND
pins as possible is also required.
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.
Output Voltage < 1.8V
The LTC3533 can operate as a buck converter with output
voltages as low as 400mV. The part is specified at 1.8V
minimum to allow operation without the requirement of a
Schottky diode; Since synchronous switch D is powered
from PV_OUT, and the R_DS(ON) will increase at low output
voltages, a Schottky diode is required from SW2 to V_OUT
Operating Frequency Selection
Higher operating frequencies allow the use of a smaller
inductor and smaller input and output filter capacitors,
thus reducing board area and component height. How-
ever, higher operating frequencies also increase the IC’s
total quiescent current due to the gate charge of the four
switches, as given by:
Buck:
I_Q = (600e – 12 • V_IN • f ) mA
Boost:
I_Q = [800e – 12 • (V_IN + V_OUT) • f ] mA
Buck/Boost: I_Q = [(1400e – 12 • V_IN + 400e – 12 •
V_OUT) • f ] mA
where f = switching frequency in Hz. Therefore frequency
selection is a compromise between the optimal efficiency
and the smallest solution size.
Table 2. Capacitor Vendor Information
SUPPLIER
PHONE
AVX
[phone]
Sanyo
[phone]
Taiyo Yuden
[phone]
TDK
[phone]
12
FAX
[phone]
[phone]
[phone]
[phone]
WEB SITE
www.avxcorp.com
www.sanyovideo.com
www.t-yuden.com
www.component.tdk.com
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