LTC3443 Datasheet, PDF (12/16 Pages) Linear Technology – High Current Micropower 600kHz Synchronous Buck-Boost DC/DC Converter

LTC3443
APPLICATIO S I FOR ATIO
above calculations in order to maintain the desired tran-
sient 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 ce-
ramic capacitors, AVX TPS series tantalum capacitors or
Sanyo POSCAP are recommended.
Input Capacitor Selection
Since the V_IN pin is the supply voltage for the IC it is
recommended to place at least a 4.7µF, low ESR bypass
capacitor.
Table 2. Capacitor Vendor Information
SUPPLIER PHONE
FAX
AVX
[phone] [phone]
Sanyo
[phone] [phone]
Taiyo Yuden [phone] [phone]
WEB SITE
www.avxcorp.com
www.sanyovideo.com
www.t-yuden.com
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)
of the NMOS to PMOS transition, improving efficiency.
Use a Schottky diode such as a Phillips PMEG2010EA or
equivalent. Do not use ordinary rectifier diodes, since the
slow recovery times will compromise efficiency. For appli-
cations with an output voltage above 4.3V, a Schottky
diode is required from SW2 to V_OUT.
Output Voltage < 2.4V
The LTC3443 can operate as a buck converter with output
voltages as low as 0.4V. The part is specified at 2.4V
minimum to allow operation without the requirement of a
Schottky diode. Synchronous switch D is powered from
V_OUT and the R_DS(ON) will increase at low output voltages,
therefore a Schottky diode is required from SW2 to V_OUT
to provide the conduction path to the output.
Output Voltage > 4.3V
A Schottky diode from SW 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 the SW1 pin
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
Additional quiescent current due to the output switches
GATE charge is given by:
Buck: 800e^–12 • V_IN • f
Boost: 400e^–12 • (V_IN + V_OUT) • f
Buck/Boost: f • (1200e^–12 • V_IN + 400e^–12 • V_OUT)
where f = switching frequency
Closing the Feedback Loop
The LTC3443 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 is
given by:
f_{FILTER_POLE} = _{2 • π •}
1
Hz
L • C_OUT
where C_OUT is the output filter capacitor.
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