LTC3532_15 Datasheet, PDF (12/16 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

LTC3532
APPLICATIO S I FOR ATIO
Table 2. Capacitor Vendor Information
SUPPLIER
AVX
Murata
Sanyo
Taiyo Yuden
TDK
WEB SITE
www.avxcorp.com
www.murata.com
www.sanyovideo.com
www.t-yuden.com
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 efficiency. Use a surface mount Schottky
diode such as an MBRM120T3 or equivalent. Do not use
ordinary rectifier diodes, since the slow recovery times
will compromise efficiency. 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.
12
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 = (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 efficiency
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 filter exhibits a double pole response, as
given by:
1
f_{FILTER—POLE} = _{2 • π •}
Hz
L • C_OUT
(in buck mode)
f_{FILTER—POLE} =
2
•
V_OUT
•
V_IN
π•
Hz
L • C_OUT
(in boost mode)
where L is in henrys and C_OUT is in farads.
The output filter zero is given by:
f_{FILTER—ZERO} =
2•π
1
• 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 =
2•π
V_IN2
• I_OUT • L • V_OUT
Hz
3532fc