LTC3530
11
3530fb
The output capacitance is usually many times larger than
the minimum value in order to handle the transient response
requirements of the converter. For 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 V
IN
is the supply voltage for the IC, as well as the
input to the power stage of the converter, it is recommended
to place at least a 10μF, low ESR ceramic bypass capaci-
tor close to the V
IN
and GND pins. 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 (V
OUT
< 4.3V), but provide a lower drop
during the break-before-make time (typically 15ns) improv-
ing effi ciency. Use a surface mount Schottky diode such as
an MBRM120T3 or equivalent. Do not use ordinary rectifi er
diodes, since the slow recovery times will compromise
effi ciency. For applications with an output voltage above
4.3V, a Schottky diode is required from SW2 to V
OUT
.
Output Voltage < 1.8V
The LTC3530 can operate as a buck converter with output
voltages as low as 0.4V. Synchronous switch D is powered
from V
OUT
and the R
DS(ON)
will increase at low output volt-
ages, therefore a Schottky diode is required from SW2 to
V
OUT
to provide the conduction path to the output. Note
that Burst Mode operation is inhibited at output voltages
below 1V typical. Note also that if V
OUT
is less than 1V,
the current limit will be 670mA (typ).
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 the parasitic lead and trace inductance.
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 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
Higher operating frequencies allow the use of a smaller
inductor and smaller input and output fi lter 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: Iq = (0.6 • V
IN
• f) mA
Boost: Iq = [0.8 • (V
IN
+ V
OUT
) • f] mA
Buck/Boost: Iq = [f • (1.4 • V
IN
+ 0.4 • V
OUT
)] mA
Table 2. Capacitor Vendor Information
SUPPLIER PHONE FAX WEB SITE
AVX (803) 448-9411 (803) 448-1943 www.avxcorp.com
Murata (814) 237-1431, (800) 831-9172 (814) 238-0409 www.murata.com
Sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com
Taiyo Yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com
TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com
APPLICATIONS INFORMATION
