LTC3532
12
3532fc
Table 2. Capacitor Vendor Information
SUPPLIER WEB SITE
AVX www.avxcorp.com
Murata www.murata.com
Sanyo www.sanyovideo.com
Taiyo Yuden www.t-yuden.com
TDK www.component.tdk.com
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 4.7μF, 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
The 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) improving 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 > 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 SGND
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: I
Q
= (0.125 • V
IN
• f) mA
Boost: I
Q
= [0.06 • (V
IN
+ V
OUT
) • f] mA
Buck/Boost: I
Q
= [f • (0.19 • V
IN
+ 0.06 • V
OUT
)] mA
where f = switching frequency in MHz. Therefore frequency
selection is a compromise between the optimal effi ciency
and the smallest solution size.
Closing the Feedback Loop
The LTC3532 incorporates voltage mode PWM control.
The control to output gain varies with operation region
(buck, boost, buck/boost), but is usually no greater than
15. The output fi lter exhibits a double pole response, as
given by:
f
FILTER
—
POLE
=
1
2•π •L•C
OUT
Hz
(in buck mode)
f
FILTER
—
POLE
=
V
IN
2•V
OUT
• π •L•C
OUT
Hz
(in boost mode)
where L is in henrys and C
OUT
is in farads.
The output fi lter zero is given by:
f
FILTER
—
ZERO
=
1
2•π •R
ESR
•C
OUT
Hz
where R
ESR
is the equivalent series resistance of the
output capacitor.
A troublesome feature in boost mode is the right-half plane
zero (RHP), given by:
f
RHPZ
=
V
IN
2
2•π •I
OUT
•L•V
OUT
Hz
APPLICATIO S I FOR ATIO
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